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1

What is the role of hydroxyappetite on stem fixation?

Hydroxyapatite may be used as surface coating on implants designed for cementless fixation.

Formula is Ca10(PO4)6 (OH)2.

Osteoconductive only

Effect—allows more rapid closure of gaps between bone and prosthesis

Bidirectional closure of space between prosthesis and bone

Osteoblasts adhere to hydroxyapatite surface during implantation and then grow toward bone.

Clinically shortens time to biologic fixation

Success requires

High crystallinity—amorphous areas of hydroxyapatite will dissolve.

Optimal thickness—a thick coating will crack and shear off.

Thickness less than 50–70 μm preferred

Surface roughness

Higher implant Ra provides increased metal-hydroxyapatite interface fracture toughness.

2

what is a proximal coated stem?

3

what is bone ongrowth technique?

Description

Prosthetic surface is prepared by blasting of the surface with an abrasive grit material. Nickname is grit blast fixation.

Grit blasting process creates microdivots—no pores, just divots. Divot diameter approximately the same size as pore hole for a porous-coated implant.

Bone grows onto rough surface, stabilizing prosthesis.

Surface roughness (Ra) (Fig. 5.13)

Ra is defined as average peak to valley on the surface of the implant.

Implant roughness determines strength of biologic fixation.

Linear relation of Ra to fixation strength

Technique

Initial rigid fixation of implant is always a press fit technique.

Femoral stem design is typically a high-angle, double-wedge taper (wedge in both coronal and sagittal planes) (Fig. 5.14).

Grit surface is extensile. Fixation strength with grit blast fixation is significantly lower than that with porous coating, and therefore the area of surface coating is greater.

There are very few cups designed with bone ongrowth surface coating.

Complication

Aseptic loosening

Stem settling occurs when initial rigid fixation is not good enough to allow osteointegration.

4

what is femoral stress shielding?

Description

Proximal femoral bone density loss observed over time in the presence of a solidly fixed implant; typically applies to cementless implants

Etiology

Stem stiffness is main factor.

Problem is modulus mismatch between stem and femoral cortex.

Factors affecting stem stiffness

Stem diameter is most important. r4

Stem stiffness approximates radius4 of stem.

Larger-diameter stems are exponentially stiffer.

Metallurgy

Co-Cr (cobalt-chrome) alloy is stiffer than titanium alloy.

Stem geometry

More stiff

Solid and round stems

Less stiff

Hollow stems, slots, flutes, taper designs

Typical scenario creating stress shielding

Large-diameter stem, of 16 mm or greater

Co-Cr alloy stem

Round, solid, cylindrical stem shaft

Extensive porous coating

Distal bone loading

5

Review the approaches for a total hip:

6

Review the surgical indications for hip surgery

(non THA procedures)

Arthroscopy

Limited indications in patients with radiographic evidence of arthritis

Preoperative joint space narrowing is negative predictor of a good clinical outcome.

THA

See Section 5, Total Hip Arthroplasty.

Hip fusion

Less frequently used as THA technology advances

Classic indications

Very young male laborer

Unilateral hip arthritis

Energy expenditure

Approximately 30% increase in energy output during ambulation

Contralateral arthritis

Abnormal gait causes arthritis in these adjacent joints in 60% of patients.

Lumbar spine

Contralateral hip

Ipsilateral knee

Symptoms of pain typically start within 25 years of hip fusion.

Hip fusion technique

Preservation of abductor complex.

Many fusions are taken down for disabling pain in adjacent joints.

Selection of fusion technique that allows successful conversion to THA.

Greater trochanteric osteotomy with lateral plate fixation is preferred technique.

Care must be taken not to injure superior gluteal nerve, which innervates abductor complex.

Fusion position

20–25 degrees of flexion

Neutral abduction

Increased back and knee pain when fusion is in abduction

Neutral or slight external rotation of 10 degrees

Fusion conversion to THA

Indications

Disabling back pain—most common

Disabling ipsilateral knee pain with instability

Excess knee stress will cause knee ligament stretch-out if fusion position is incorrect.

Disabling contralateral hip pain

Function after conversion to THA

Hip function and clinical results directly related to integrity of abductor complex

Preoperative electromyogram of gluteus medius may be helpful.

When hip abductor complex nonfunctional

Severe lurching gait results

Very high risk for instability; may require constrained acetabular component

Resection arthroplasty

Indications

Incurable infection

Patients are most often immunocompromised.

Recurrent periprosthetic THA infection

Failed hip fusion with infection

Chronic destructive septic arthritis

Noncompliant patient with recurrent THA dislocation

Nonambulator

Intractable pain from arthritis

Hip fracture with open decubitus ulcers

Significant contracture interfering with hygiene and posture

Failed hip fusion in patient with prior major trauma to hip and/or pelvis

Soft tissue loss to hip region precludes successful placement of THA.

Neurologic injury to extremity precludes successful function of THA.

Hemiarthroplasty

Not routinely used in the treatment of arthritis and is relegated to specific limited role

Fracture treatment in low-demand elderly patient

Best indication—displaced subcapital hip fracture with little or no prior history of symptomatic hip arthritis

Patient not able to comply with standard THA precautions (dementia)

High risk for dislocation (Parkinson disease)

Advantages

Reduced surgical time

Stability

Maximizes head-neck ratio.

Large-diameter ball requires more distance to travel before dislocation.

Suction fit provided by labrum (may be negated if labrum and capsule resected)

Disadvantages

Groin pain in active individuals

Increased risk for need for conversion to THA in active individual due to acetabular erosion

Protrusio deformity may result, particularly if osteoporosis present

7

Non-operative treatment for hip arthritis

Activity modification

Reduction of impact-loading exercises

Reduction of weight

Avoidance of stairs, inclines, squatting

Physical therapy

Nonnarcotic medications

NSAIDs

Evidence does not support the use of glucosamine sulfate.

Joint injections

Corticosteroid—antiinflammatory treatment

Hyaluronate

Backbone of proteoglycan chain of articular cartilage

No strong evidence to support use in the hip

Not approved by FDA for hip use in United States

Assist device (cane or crutch)

Opposite hand of affected hip

8

common causes of pain after THA:

Start-up pain is the most common initial presentation of loosening.

Groin pain indicates a loose acetabular cup.

Thigh pain indicates a loose femoral stem.

Infection must always be ruled out as a cause of pain.

Anterior iliopsoas impingement and tendinitis may be the cause of groin pain in THA when a prominent or malpositioned cup is present and no other causes can be found.

9

Review Rubash Screw Quadrants

Posterior-superior quadrant is the safe zone for acetabular screw placement. This is preferred zone for screw placement.

Anterior-superior quadrant is considered the zone of death. Screws and/or drill that penetrate too far risk laceration of the external iliac artery and veins.

If a major vessel injury occurs during screw placement, the hip wound should be immediately packed tight. Without closure of the hip wound, an anterior pelvic incision is made to gain proximal control of the bleeding artery. Repair of the bleeding source is then addressed.

10

What is the difference between cavitary and segmental loss?

Cavitary deficiency is a loss of cancellous bone without compromise of main structural bone support.

Segmental deficiency is loss of main bony support structures.

Acetabular rim

Acetabular column

Medial wall

Combined deficiencies

Well-fixed cementless implant with osteolytic defect

Can be treated with débridement, bone grafting, and bearing component exchange without revision of the cup.

Contraindications to this approach are a poorly positioned cup, poor implant design, an ongrowth fixation surface, or damaged locking mechanism.

Significance of bone defects

Major segmental bone deficiencies require a reconstruction cage, structural bone graft, or modular porous metal augments.

A structural bone graft (a graft that reconstructs a segmental defect) alone without a cage has a high loosening rate.

11

what are the reconstruction options for acetabular revision?

Cementless porous biologic fixation is preferred.

A cemented cup with impaction bone grafting is used more frequently outside of North America.

Hemispheric porous cup with screws is most common solution.

Must have at least two-thirds of rim and a reasonable initial press fit to work

Requires at least 50% contact with host acetabular bone

Recommended cup replacement is to re-create the native center of rotation.

Cup placement should be inferior and medial (i.e., low and in).

Lowest joint reactive forces

Cup placement superior and lateral (i.e., up and out) is not recommended.

Highest joint reactive forces

Higher wear and component loosening

Filling of cavitary deficiencies with particulate bone graft.

Acetabular porous metal wedge augmentation is an acceptable adjuvant to hemispheric cup to achieve stability and fixation when necessary.

Reconstruction cage (Fig. 5.20)

Used when segmental bone deficiencies prevent initial rigid fixation of a hemispheric porous cup in desired position.

Bone graft

Cage placement is against acetabulum and pelvis. Bone graft is placed behind cage.

Particulate graft preferred

Bulk support allograft when needed

Acetabular cup insertion

Acetabular cup is cemented into reconstruction cage.

Mid- to long-term failure rates using this technique are significant because of mechanical loosening and/or breakage of the cage as a result of lack of biologic fixation. Many surgeons have abandoned this technique in favor of porous metal augments, cup-cage constructs, and custom triflange cups.

Modular porous metal construct (Fig. 5.21)

Increasingly being used for cases of severe bone loss

May allow achievement of mechanical stability and osseointegration when less than 50% host bone contact is available for a hemispherical implant.

Can help facilitate restoration of the hip center of rotation by filling superior defects

Different highly porous metal options available, including tantalum (75% porous by volume).

Intraoperative flexibility to match defects

Revision cup may be combined with a cage in a so-called cup-cage construct to improve initial stability and fixation

Custom triflange cup (Fig. 5.22)

Severe cases of bone loss where defect-matching techniques are limited

Decision to use is made preoperatively as this cup is custom made for each patient on the basis of a CT scan.

Requires several weeks to manufacture

12

Review the bone defects for revision of the femur

Cavitary deficiency is loss of endosteal bone. Cortical tube remains intact.

Endosteal ectasia is a form of cavitary deficiency in which the outer cortex has increased in diameter as a result of mechanical irritation by a loose femoral stem.

Segmental deficiency is a loss of part of the cortical tube in the form of either holes in or complete loss of a portion of the proximal femur.

Combined deficiencies

Significance of bone defects

Revision femoral stem must bypass the most distal defect.

New implant must bypass most distal cortical defect by a minimum of two cortical diameters. Otherwise there is an increased risk for fracture at the tip of the stem.

The revision stem must prevent bending movements from passing through the region of the cortical hole, which is a weak point.

Extensive metadiaphyseal bone loss and a nonsupportive diaphysis (Paprosky type IV classification) require a femoral replacement endoprosthesis or an allograft-prosthetic composite.

Fixation revision of femur

Cementless porous biologic fixation is preferred.

Cemented revision stems without impaction bone grafting have high failure rates at intermediate term and limited indications in the revision setting.

Extensively porous-coated cylindrical long-stem prosthesis

Monoblock stem typically made of Co-Cr

Achieves fixation in the diaphysis

Longer stems may be bowed, and engagement of the stem in the canal will dictate anteversion.

Stem should bypass defects and be long enough to achieve initial rigid fixation.

Extensively grit-blasted stem with splines also an accepted solution

Minimum of 4 cm of diaphyseal bone required

Becoming less popular due to technical difficulty in use, risk for fracture, and thigh pain with large stiff implants

Tapered fluted implant

Monoblock or modular stem made of more flexible titanium with a roughened surface

Achieves stability in the diaphysis

Taper design provides axial stability, and flutes provide rotational control.

Modular junctions allow for freedom in component anteversion and leg length but may increase the risk of breakage.

May obtain adequate stability and fixation with less than 4 cm of diaphyseal bone

Becoming more popular due to ease of use and ability to restore biomechanics through modularity

Cemented revision stem

High intermediate-term failure rate

Reasonable consideration in patients with irradiated bone

Acceptable for use in very elderly or very low-demand patient when immediate full weight bearing is needed

Impaction grafting technique

Acceptable revision technique with greater popularity outside North America

Surgical technique

Distal cement restrictor placed into diaphysis

Particulate allograft bone (fresh frozen bone recommended) impacted into endosteal canal. Bone is impacted around a femoral stem trial

Polished tapered stem cemented into impacted allograft bone

Polished tapered stem allowed to settle slightly within cement. Mechanical load forces are transmitted as compression forces upon allograft bone.

Allograft heals to endosteal bone.

Cement stays interdigitated with allograft.

Endosteal bone is restored.

Indications

Used to reconstitute cortical bone when there is significant cortical ectasia

Cortical tube must be intact. Small cortical defects can be covered with an external mesh or allograft strut.

Bone must not be devascularized during process of covering hole

Complications

Most common complications are fracture and subsidence.

Choice of allograft and morcelization technique are important factors affecting success.

Segmental bone deficiency of femur

Cortical holes are reinforced with allograft cortical struts secured with cerclage cables (or wires).

Proximal cortical deficiencies may be restored with modular metallic endoprosthetic segments (proximal femur replacement) or with a bulk support allograft.

Proximal allograft technique (allograft-prosthesis composite, or APC)

Revision stem cemented into proximal allograft

Allograft connected to host femur with a step cut or through an intussusception (telescoping) technique.

Allograft held to native femur with cables, plate, and/or allograft cortical strut.

13

what to do when intra-operative THA fracture?!?

Highest risk is with cementless implants.

Acetabular fracture

Most common reason for fracture is underreaming.

Underreaming of 2 mm or more associated with higher fracture risk

Cup may be left in place if stable, and additional screws used to enhance fixation.

An unstable cup needs to be revised and may require a posterior column plate.

Femoral fracture

A longitudinal split in the calcar encountered during implantation of a tapered, proximally coated stem may be treated with stem removal, cabling, and reinsertion.

If this procedure does not result in a stable implant, a stem that bypasses the fracture and achieves diaphyseal fixation may be needed.

14

Review the Vancover Fracture Classification

15

Nerve Injury and THA

Involved nerves: 80% sciatic nerve, 20% femoral nerve

Compression is most common pathologic mechanism of injury.

Of patients who have a nerve injury after primary THA, only 35%–40% will have recovery to normal strength.

Sciatic nerve travels closest to acetabulum at the level of ischium.

During surgery, the most common reason for sciatic nerve injury is errant retractor placement causing excess compression to nerve.

Peroneal nerve division is most often involved because this part of nerve is closest to acetabulum.

Risk factors for nerve injury

Female gender

Posttraumatic arthritis

Revision surgery

Developmental dysplasia of the hip

Risk for sciatic nerve palsy increases with leg lengthening of more than 3–5 cm.

Postoperative functional footdrop

Clinical scenario—patient sits in chair after surgery and experiences footdrop.

With hip flexed 90 degrees in chair, there is too much tension on sciatic nerve.

Treatment—patient returned to bed.

Hip placed in extension (bed flat)

Knee flexed on one or two pillows

This position provides least tension on sciatic nerve.

Postoperative hematoma

A hip hematoma from anticoagulation can cause sciatic nerve palsy.

Compression is mechanism of injury.

Treatment is immediate evacuation of hematoma.

16

specific factors associated with THA complications

Sickle cell disease

Associated with early prosthetic loosening

Mechanism is extended bone infarct disease.

Higher risk of periprosthetic joint infection.

Psoriatic arthritis

Associated with higher periprosthetic infection rate

Ankylosing spondylitis

Associated with higher risk for heterotopic ossification (HO)

Hip hyperextension due to fixed pelvic deformity can lead to a higher anterior dislocation rate.

Parkinson disease

Higher dislocation rate

Higher perioperative mortality

Higher perioperative medical complications

Higher reoperation rate

Paget disease

Increased blood loss

Good results may still be obtained with cementless fixation.

Dialysis

Higher risk of infection and loosening

Fat emboli syndrome

Occurs with femoral stem insertion

Fat and bone marrow emboli are pressurized into bloodstream.

Intraoperative hypotension, hypoxia, mental status changes, and petechial rash are hallmark findings.

Treatment is volume and respiratory support.

17

HO and THA

Small amounts of clinically insignificant HO are common and likely present in a majority of patients undergoing THA.

Risk factors: male gender, ankylosing spondylitis, hypertrophic subtype of arthritis, posttraumatic arthritis, head injury, and history of HO

Prevention

Careful handling of soft tissues

Prophylaxis with oral indomethacin or radiation therapy (700–800 Gy) within 24 hours prior to surgery or 72 hours after surgery

Treatment

No effective treatment in early postoperative period once the process has started

Indication for surgical resection is significant loss of motion.

Process should appear mature and stable on serial radiographs before resection is undertaken.

Heterotopic bone may recur after operative resection.

18

hip psoas after THA

Underrecognized cause of groin pain after THA

Discomfort with resisted hip flexion and straight-leg raise

Cross-table lateral radiograph or CT may show a retroverted cup or anterior overhang of the acetabular component.

An injection may be used to confirm the diagnosis.

Treatment is with release or resection of the iliopsoas tendon, alone or in combination with acetabular revision for an anterior overhanging component.

Acetabular revision is more predictable for groin pain resolution in patients with 8 mm or more of anterior acetabular component prominence.

19

Implant stiffness:

youngs module

20

Coefficient of friction:

bearing materials

21

review the treatment for THA hip instability

Each case of hip dislocation is unique. There is not one common treatment.

In each case the following issues, as previously described, should be assessed:

Component design

Component alignment

Soft tissue tension

Soft tissue function

Spinopelvic mobility

Clinical review of dislocating event important

Was dislocation at extreme end range or within the range for usual activities of daily living?

Patient’s cognition—impaired versus normal

Clinical examination

Determination of where THA starts to lever and sublux

Radiographic review

Implant design and position should be scrutinized.

Sitting lateral and standing lateral x-rays may be obtained to assess spinopelvic relationship and mobility.

Initial treatment for dislocated THA

Two-thirds of patients with first-time THA dislocation early in the postoperative period can be successfully treated with closed measures.

Closed reduction

Sedation or anesthesia preferred to minimize soft tissue trauma

During closed reduction, hip is taken through full range to assess position of dislocation.

Subluxation is identified as being either within patient’s activities of daily living or at extreme end range.

Posterior hip dislocation

In supine position, the leg lies in internal rotation, adduction, and shortened position.

Reduction maneuver for posterior hip dislocation

Flexion to 80–90 degrees

Internal rotation

Adduction

Distraction

Anterior hip dislocation

In supine position, the leg lies in external rotation, slight abduction, and slightly shortened position.

Reduction maneuver for anterior hip dislocation

Extension

External rotation

Slight abduction

Distraction

Postreduction treatment

Education—hip precautions

Immobilization of joint (usually 6 weeks)

Spica brace/cast

Knee immobilizer—to keep patient from putting hip in a compromised position

Physical therapy

Focus on strength, balance, agility, coordination

Optimization of medical conditions

Surgical treatment

Surgical options

Implant revision

Greater trochanter advancement

Constrained acetabular socket (cup)

Conversion to bipolar hemiarthroplasty

Resection arthroplasty

Rule 1 for surgical treatment: If any implant component is malaligned, it needs to be changed.

May require complete hip revision

Goals of component revision

Maximizes head/neck ratio to increase primary arc range.

No neck skirts or acetabular hoods

Accurate component alignment

Re-creates center of hip rotation and head offset.

Stabilizes greater trochanter (if possible) if it is detached.

Dual-mobility cups may allow for use of a larger femoral head and improve range of motion before impingement.

Greater trochanter advancement (also known as Charnley tensioning) (Fig. 5.62)

Technique is to perform trochanteric osteotomy, advance the greater trochanter distally on the lateral femur, and resecure it with claw, cables, and/or wires.

With distal advancement of the greater trochanter, the abductor complex is tensioned more tightly, thereby increasing hip compression forces.

Requirements

No component malalignment

Adequate distal bone surface for bony fixation and bone healing

Intact superior gluteal nerve

Constrained PE liner

A constrained PE liner encloses the femoral head and mechanically prevents hip from distracting out of socket.

Reserved as a last resort for the patient who has experienced multiple dislocations with soft tissue dysfunction and appropriately positioned implants.

Best indications

Elderly patient (i.e., low demand) with normal component alignment

Abductor deficiency/dysfunction

Central neurologic decline

Revision THA with reconstruction cage (Fig. 5.63)

Significant soft tissue dissection and potential muscle dysfunction with cage placement

Contraindication

Cup malposition

Failure mechanisms

Because a constrained cup significantly reduces primary arc range (to as low as 60–70 degrees), the cup is exposed to more frequent and more intense lever range forces. With repetitive loading, the constrained cup will fail via two different mechanisms.

The PE deforms at the edges of the socket and the hip dislocates.

The PE does not deform. In this case the levering forces are then transmitted to the acetabular prosthetic-bone interface, resulting in mechanical loosening of the cup.

In the patient with failed constrained PE liner, the rate of subsequent failure is high if revision involves use of another constrained PE liner.

Conversion to bipolar hemiarthroplasty

Rarely done and should be reserved for unusual circumstances

Technique: removal of acetabular component; reaming of remaining bone to a hemisphere; press fitting of bipolar ball to rim of acetabulum (minimizes risk for medial migration of head).

Requirements

Fully intact acetabular bone

No segmental rim deficiencies; otherwise the bipolar ball will dislocate

Good bone density

Rim fit technique

Advantages

Maximizes fully head/neck ratio

Bipolar construct has a little more inherent stability than monopolar ball.

Disadvantages

Groin pain—metal articulating on bone (Fig. 5.64)

Medial migration of head developing into protrusio deformity

Accelerated PE wear

Larger overall PE wear surface area

Resection arthroplasty

Indications

Nonambulatory patient

Neurologic deficits in which stability cannot be achieved

Recurrent/ongoing periprosthetic infection

Drug-seeking behavior with purposeful voluntary dislocations

22

what is the incidence of hip dislocation

Primary THA—typically 1%–2%

Revision THA—typically 5%–7%

Highest incidence of dislocation

THA in the elderly patient (>80 years) for failed ORIF of femoral neck fracture—reasons:

Muscular weakness

Mental compromise

Loss of balance and coordination

23

What are the risk factors for dislocation

Female gender

THA for osteonecrosis

Posterolateral approach

Smaller femoral head size

Greater trochanter nonunion

Revision THA

Obesity

Alcoholism

Neuromuscular conditions

24

Evaluating hip instability

Assessment of a dislocating THA evaluates:

Component design

Component alignment

Soft tissue tension

Soft tissue function

Spinopelvic alignment and postural pelvic positioning

Component design

Prosthetic ROM consists of two parts, primary arc range (Fig. 5.46) and lever range (Fig. 5.47).

Primary arc range

Primary arc range is controlled by the head/neck ratio (Fig. 5.48).

Ratio of head diameter to neck diameter is head/neck ratio.

Best stability is achieved by maximizing head/neck ratio (Fig. 5.49).

Additions to acetabulum and/or femoral neck decrease primary arc range.

Neck skirt (also known as femoral head collar on femoral stem)

Decreases head/neck ratio

Acetabular hoods

Decrease primary arc range (Fig. 5.50)

Acetabular constrained cups

Markedly decrease primary arc range (Fig. 5.51)

Rule—constrained cups should be avoided as much as possible.

Lever range

Range allowed as hip starts to lever out of socket

Lever range is controlled by head radius (Fig. 5.52).

A large head has higher excursion distance and is more stable.

Most stable construct is a bipolar hemiarthroplasty (two pivot points).

Best range in THA

High primary arc range

Maximized head/neck ratio

No additions to cup or neck

High excursion distance

Large-diameter head

Component alignment

Primary arc range must be centered within patient’s functional hip range (Fig. 5.53).

Component malalignment does not decrease primary arc range.

Placement of components in a malaligned position results in a stable side and an unstable side of the functional hip range.

Implant positioning in THA

Cup anteversion: 20–30 degrees (Fig. 5.54)

Cup theta (ϴ) angle (also known as coronal tilt): 35–40 degrees (Fig. 5.55)

Stem anteversion: 10–15 degrees (Fig. 5.56)

Cup malposition

Retroversion—risk is posterior dislocation.

Excess anteversion—risk is anterior dislocation.

High ϴ-angle (vertical cup)—risk is posterior-superior dislocation.

Low θ-angle (horizontal cup)—risk is inferior dislocation.

Stem malposition

Retroversion—risk is posterior dislocation.

Excess anteversion—risk is anterior dislocation.

Soft tissue tension

Abductor complex is key to hip stability.

Consists primarily of gluteus medius and gluteus minimus muscles

Prosthetic implant design and positioning must maintain/restore proper abductor hip tension.

Restoration of abductor tension achieved by the following (Fig. 5.57):

Restored normal hip center of rotation

Restored head offset

Restored femoral neck length

Problems with reduced hip offset

Weakened abductor complex

Increased joint reaction force (decreased abductor lever arm)

Presence of Trendelenburg sign

Gluteus medius lurch with walking

Increased risk for dislocation

Problems with short neck length

Short neck length occurs when a low neck cut is made, a short prosthetic neck length is used, or both.

Shortens abductor muscle length, resulting in abductor weakness

Decreases hip offset, which also weakens abductor complex

Results in bony impingement of greater trochanter against pelvis during hip range (Fig. 5.58)

Causes pain

Allows hip levering and increases risk for dislocation

Shortens leg length

Problems with restoring neck length by using a long head

A short femoral neck cut can be compensated with an extra-long prosthetic neck length. However, a long neck length requires a skirt (Fig. 5.59).

A skirt decreases primary arc range.

Increases risk for dislocation

If the prosthetic neck-shaft angle is lower than that of native hip, the addition of neck length will excessively increase neck offset.

This can cause trochanter bursitis and chronic lateral hip pain (Fig. 5.60).

Narrow-offset femoral stem design

A femoral stem with an offset designed with a narrower angle than the native hip will reduce hip offset. This reduces abductor tension and increases risk for hip dislocation.

A narrow-offset stem can be compensated for by with a longer femoral head length (i.e., neck length). This creates two potential problems.

Addition of neck length to restore offset will excessively lengthen the leg.

Addition of a long neck requires a skirt, which decreases primary arc range and increases risk for dislocation.

Greater trochanteric escape (Fig. 5.61)

Greater trochanteric escape occurs when the greater trochanter pulls away from the proximal femur.

Usually a result of failed trochanter fixation after revision THA

Can also occur from trauma (usually associated with osteolysis in greater trochanter)

Successful reattachment is difficult and often fails.

Problems

Because the hip abductor complex attaches to the greater trochanter, trochanteric escape results in a loss of hip compression and increases risk for hip dislocation.

There is greater external and internal hip rotation because the greater trochanter no longer restricts rotation range. This also increases the risk for dislocation because the hip can more easily approach and exceed lever range.

The greater trochanter fragment can impinge between the hip and pelvis, causing hip levering.

Treatment

Maximized head/neck ratio.

No neck skirts and no acetabular hoods

Resection of greater trochanter fragment to prevent impingement levering.

Constrained acetabular cup is last resort.

Soft tissue function

The soft tissues about the hip are controlled by several body systems. All are integrated to provide hip stability. The three main factors controlling soft tissue function are:

Central nervous system

Peripheral nervous system

Local soft tissue integrity (surrounding hip region)

CNS mechanisms causing disruption to hip function and increasing risk for dislocation

Muscle dysfunction

Sensory impairment

Impaired coordination

Impaired balance

Cognitive loss of restraint (i.e., compliance/memory)

CNS conditions affecting hip function

Cerebral dysfunction

Stroke, seizure, CNS disease

Cerebellar dysfunction

Balance/coordination

Delirium

Medications, withdrawal phenomenon

Dementia

Psychiatric problems

Psychosis, addiction

Peripheral nervous system mechanisms causing disruption to hip function and thereby increasing risk for dislocation

Muscle dysfunction

Sensory impairment

Pain

Peripheral nervous system conditions affecting hip

Spinal stenosis

Radiculopathy

Neuropathy

Paralysis/paresis

Local soft tissue integrity mechanisms causing disruption to hip function and increasing risk for dislocation

Muscle dysfunction

Soft tissue dysfunction (other than muscle)

Soft tissue loss

Skeletal deformity

Example: patients with ankylosing spondylitis have increased risk for anterior dislocation.

Local soft tissue conditions affecting hip function

Trauma

Soft tissue loss

Myoligamentous disruption

Deconditioning

Poor health

Aging process

Irradiation

Radiation fibrosis with soft tissue contraction

Dysplasia

Musculoskeletal hypoplasia

Osteolysis

Bone loss

Myotendinous disruption

Collagen abnormalities

Clinical hyperelasticity

Myopathy

Malignancy

Infection

Spinopelvic alignment

Increasingly recognized as a potential contributing factor to instability following THA

Patients with fixed spinopelvic alignment in sitting and standing positions are at higher risk of dislocation.

Patients with lumbar spine disease and previous lumbar spine surgery may have reduced spinopelvic motion and are at higher risk for instability.

25

Changing to a ceramic head with a used femoral head

Assessment of a dislocating THA evaluates:

Component design

Component alignment

Soft tissue tension

Soft tissue function

Spinopelvic alignment and postural pelvic positioning

Component design

Prosthetic ROM consists of two parts, primary arc range (Fig. 5.46) and lever range (Fig. 5.47).

Primary arc range

Primary arc range is controlled by the head/neck ratio (Fig. 5.48).

Ratio of head diameter to neck diameter is head/neck ratio.

Best stability is achieved by maximizing head/neck ratio (Fig. 5.49).

Additions to acetabulum and/or femoral neck decrease primary arc range.

Neck skirt (also known as femoral head collar on femoral stem)

Decreases head/neck ratio

Acetabular hoods

Decrease primary arc range (Fig. 5.50)

Acetabular constrained cups

Markedly decrease primary arc range (Fig. 5.51)

Rule—constrained cups should be avoided as much as possible.

Lever range

Range allowed as hip starts to lever out of socket

Lever range is controlled by head radius (Fig. 5.52).

A large head has higher excursion distance and is more stable.

Most stable construct is a bipolar hemiarthroplasty (two pivot points).

Best range in THA

High primary arc range

Maximized head/neck ratio

No additions to cup or neck

High excursion distance

Large-diameter head

Component alignment

Primary arc range must be centered within patient’s functional hip range (Fig. 5.53).

Component malalignment does not decrease primary arc range.

Placement of components in a malaligned position results in a stable side and an unstable side of the functional hip range.

Implant positioning in THA

Cup anteversion: 20–30 degrees (Fig. 5.54)

Cup theta (ϴ) angle (also known as coronal tilt): 35–40 degrees (Fig. 5.55)

Stem anteversion: 10–15 degrees (Fig. 5.56)

Cup malposition

Retroversion—risk is posterior dislocation.

Excess anteversion—risk is anterior dislocation.

High ϴ-angle (vertical cup)—risk is posterior-superior dislocation.

Low θ-angle (horizontal cup)—risk is inferior dislocation.

Stem malposition

Retroversion—risk is posterior dislocation.

Excess anteversion—risk is anterior dislocation.

Soft tissue tension

Abductor complex is key to hip stability.

Consists primarily of gluteus medius and gluteus minimus muscles

Prosthetic implant design and positioning must maintain/restore proper abductor hip tension.

Restoration of abductor tension achieved by the following (Fig. 5.57):

Restored normal hip center of rotation

Restored head offset

Restored femoral neck length

Problems with reduced hip offset

Weakened abductor complex

Increased joint reaction force (decreased abductor lever arm)

Presence of Trendelenburg sign

Gluteus medius lurch with walking

Increased risk for dislocation

Problems with short neck length

Short neck length occurs when a low neck cut is made, a short prosthetic neck length is used, or both.

Shortens abductor muscle length, resulting in abductor weakness

Decreases hip offset, which also weakens abductor complex

Results in bony impingement of greater trochanter against pelvis during hip range (Fig. 5.58)

Causes pain

Allows hip levering and increases risk for dislocation

Shortens leg length

Problems with restoring neck length by using a long head

A short femoral neck cut can be compensated with an extra-long prosthetic neck length. However, a long neck length requires a skirt (Fig. 5.59).

A skirt decreases primary arc range.

Increases risk for dislocation

If the prosthetic neck-shaft angle is lower than that of native hip, the addition of neck length will excessively increase neck offset.

This can cause trochanter bursitis and chronic lateral hip pain (Fig. 5.60).

Narrow-offset femoral stem design

A femoral stem with an offset designed with a narrower angle than the native hip will reduce hip offset. This reduces abductor tension and increases risk for hip dislocation.

A narrow-offset stem can be compensated for by with a longer femoral head length (i.e., neck length). This creates two potential problems.

Addition of neck length to restore offset will excessively lengthen the leg.

Addition of a long neck requires a skirt, which decreases primary arc range and increases risk for dislocation.

Greater trochanteric escape (Fig. 5.61)

Greater trochanteric escape occurs when the greater trochanter pulls away from the proximal femur.

Usually a result of failed trochanter fixation after revision THA

Can also occur from trauma (usually associated with osteolysis in greater trochanter)

Successful reattachment is difficult and often fails.

Problems

Because the hip abductor complex attaches to the greater trochanter, trochanteric escape results in a loss of hip compression and increases risk for hip dislocation.

There is greater external and internal hip rotation because the greater trochanter no longer restricts rotation range. This also increases the risk for dislocation because the hip can more easily approach and exceed lever range.

The greater trochanter fragment can impinge between the hip and pelvis, causing hip levering.

Treatment

Maximized head/neck ratio.

No neck skirts and no acetabular hoods

Resection of greater trochanter fragment to prevent impingement levering.

Constrained acetabular cup is last resort.

Soft tissue function

The soft tissues about the hip are controlled by several body systems. All are integrated to provide hip stability. The three main factors controlling soft tissue function are:

Central nervous system

Peripheral nervous system

Local soft tissue integrity (surrounding hip region)

CNS mechanisms causing disruption to hip function and increasing risk for dislocation

Muscle dysfunction

Sensory impairment

Impaired coordination

Impaired balance

Cognitive loss of restraint (i.e., compliance/memory)

CNS conditions affecting hip function

Cerebral dysfunction

Stroke, seizure, CNS disease

Cerebellar dysfunction

Balance/coordination

Delirium

Medications, withdrawal phenomenon

Dementia

Psychiatric problems

Psychosis, addiction

Peripheral nervous system mechanisms causing disruption to hip function and thereby increasing risk for dislocation

Muscle dysfunction

Sensory impairment

Pain

Peripheral nervous system conditions affecting hip

Spinal stenosis

Radiculopathy

Neuropathy

Paralysis/paresis

Local soft tissue integrity mechanisms causing disruption to hip function and increasing risk for dislocation

Muscle dysfunction

Soft tissue dysfunction (other than muscle)

Soft tissue loss

Skeletal deformity

Example: patients with ankylosing spondylitis have increased risk for anterior dislocation.

Local soft tissue conditions affecting hip function

Trauma

Soft tissue loss

Myoligamentous disruption

Deconditioning

Poor health

Aging process

Irradiation

Radiation fibrosis with soft tissue contraction

Dysplasia

Musculoskeletal hypoplasia

Osteolysis

Bone loss

Myotendinous disruption

Collagen abnormalities

Clinical hyperelasticity

Myopathy

Malignancy

Infection

Spinopelvic alignment

Increasingly recognized as a potential contributing factor to instability following THA

Patients with fixed spinopelvic alignment in sitting and standing positions are at higher risk of dislocation.

Patients with lumbar spine disease and previous lumbar spine surgery may have reduced spinopelvic motion and are at higher risk for instability.

26

why ceramic heads are best/most common

Ceramic on highly crosslinked polyethylene (HCLPE) has become the most popular bearing option in North America.

Rates of osteolysis have fallen dramatically with the widespread adoption of HCLPE.

Concerns about trunnion corrosion have reduced the use of Co-Cr femoral heads in favor of ceramic heads.

Although polyethylene (PE) wear debris has historically been the main culprit of osteolysis, metal debris from metal-on-metal bearings and trunnion corrosion may have been a more common reason for osteolysis in the past decade.

27

poly as a hip bearing

Ultra-high-molecular-weight polyethylene (UHMWPE) was introduced in the 1960s by Sir John Charnley.

HCLPE was introduced in the late 1990s to reduce wear and osteolysis.

Cross-linking of poly chains made by irradiation of PE.

Disadvantage of HCLPE is reduction in mechanical properties and can lead to catastrophic failure of the implant

28

Review highly cross linked Polyethylene

HCLPE

Irradiation of PE ruptures PE bond, creating free radicals.

Free radicals can rebond via two different pathways.

In presence of oxygen (i.e., air), free radicals can bond with oxygen, resulting in PE chain scission. This is termed oxidized PE. Irradiation of PE must be done in an inert environment (i.e., with no oxygen).

In absence of oxygen, the free radical bonds with an adjacent chain to create a cross-link. This is termed cross-linked PE.

Dose of irradiation

Requires high-dose irradiation, 5–15 Mrad (10 Mrad = 100 kGy)

The higher the dose of irradiation, the greater number of free radicals generated.

Problem—residual free radicals after cross-linking do remain. This is an oxidation risk.

Free radical elimination

Postirradiation thermal treatment of the polymer is required to reduce remaining free radicals to ensure the oxidative stability of joint implants in the long term.

Annealing—heating below the melting point—preserves mechanical properties but does not completely eliminate free radicals.

Melting—heating above the melting point—decreases mechanical properties owing to loss of crystallinity but eliminates free radicals.

Vitamin E, an antioxidant, may be added to PE as another method to stabilize free radicals

29

what are the ways to remove free radicals after radiation?

Postirradiation thermal treatment of the polymer is required to reduce remaining free radicals to ensure the oxidative stability of joint implants in the long term.

Annealing—heating below the melting point—preserves mechanical properties but does not completely eliminate free radicals.

Melting—heating above the melting point—decreases mechanical properties owing to loss of crystallinity but eliminates free radicals.

Vitamin E, an antioxidant, may be added to PE as another method to stabilize free radicals

30

Review the physical property of UHMWPE

PE in a manufactured implant exists in two forms (i.e., phases; Fig. 5.26).

Crystalline phase

Provides mechanical strength to PE

Amorphous phase

Only amorphous regions of PE cross-link.

PE properties—crystallinity

Optimum crystallinity 45%–65%

Decreased crystallinity (<45%)

Decreases mechanical properties

PE more prone to macroscopic failure (i.e., cracks)

Increased crystallinity (>65%)

The large crystalline phase leaves a very small amorphous phase.

The greatly reduced amorphous region is more susceptible to chain scission oxidation.

Creates significant increase in particulate debris