7.10 Total hip replacement: modes of failure

THR is a successful treatment for osteoarthritis of the hip in the vast majority of patients. It reliably abolishes pain and restores function. Complications can and do arise, however, and the purpose of this chapter is to discuss the modes of failure of THR. Infection, loosening, implant failure, and periprosthetic fractures will be discussed.

Deep infection is a devastating complication of THR often requiring revision surgery. The incidence of deep infection after THR is 1–2%. Risk factors include diabetes, obesity, sickle cell disease, rheumatoid disease, HIV infection, cancer, and immunosuppressant therapy.

The most common organisms include Staphylococcus aureus, Staphylococcus epidermidis and coliforms. These pathogens are usually commensal organisms living on the host but can be transmitted by medical staff. Methicillin resistant Staphylococcus aureus infections (MRSA) are rare and are usually seen in patients with associated comorbidity who have had prolonged antibiotic therapy and long hospital admissions. In patients with sickle cell disease, other organisms such as Salmonella are seen. In HIV, the infections can be atypical and include fungi as well as bacteria.

Some bacteria produce a glycocalyx that acts as a biofilm. This biofilm helps the bacteria to adhere to the implant, inhibits the action of macrophages, and protects the bacteria from antibiotics. Glycocalyx-producing bacteria are harder to eradicate than conventional bacteria and almost always require two-stage revision surgery.

Broadly speaking infection can be classified as acute, chronic, or delayed acute (see Table 7.10.1). Acute infections present within the first postoperative month. Chronic infections probably occur at the time of surgery but present some months or years later. There may be little to find clinically. Delayed acute infections are those that arise via haematogenous spread some months or years after implantation of the THR and are best treated as acute infections.

Measures that can be taken to avoid infection are subdivided into patient factors and hospital factors. Patients should be optimized prior to surgery. Diabetes should be well controlled, sites of potential infection should be screened, e.g. urine, and any pending dental treatment or ‘dirty’ surgery, e.g. prostatectomy, should be completed prior to THR. Hospital factors include using prophylactic antibiotics, laminar flow theatres, exhaust suits, minimizing the number of staff in theatre, careful soft tissue handling, antibiotic impregnated cement, wound lavage, and expert postoperative wound care.

Acute infections are normally associated with pyrexia, wound erythema, and purulent discharge. The presentation of chronic infections is more varied. Patients will often give a history of pain, particularly if the implants are loose. Clinically the picture ranges from a well healed, cool scar to a hot, erythematous thigh with draining sinuses.

Standard investigations include plain radiographs, inflammatory markers (C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR)) and hip aspiration. Characteristics of infection seen on plain radiographs include loosening, endosteal scalloping, and periostitis (Figure 7.10.1). Technetium-99 bone scans are hot in the majority of cases of infection but can also be positive in aseptic loosening. White cell scans using indium-111 have higher specificity for infection. Often it is difficult to distinguish aseptic from septic loosening. If a THR is deeply infected it is important to attempt to identify the offending organisms and their sensitivities prior to any revision surgery so an antibiotic regimen can be planned in advance. It is also essential to obtain tissue samples send at the time of revision surgery to confirm the suspected diagnosis of infection. These must be obtained prior to the administration of antibiotics perioperatively.

The treatment strategy depends on the severity of symptoms, suitability of the patient for revision surgery and functional expectations of the patient. The goal in some patients is simply suppression of the infection and in others it is complete eradication of infection.

Patients who are not suitable for revision surgery, who have a solidly fixed prosthesis, and have a sensitive organism may simply be treated with long-term oral antibiotics.

Acutely infected THRs (including the acute delayed infections) can be salvaged with wound excision, aggressive debridement, extensive lavage, exchange of modular components, e.g. the head, acetabular liner, screws, hole eliminators, non-absorbable sutures, etc., and primary wound closure over a suction drain. Appropriate intravenous antibiotics should be started as soon as samples have been obtained for microbiology and continued until the inflammatory markers have returned to normal. The duration of treatment is usually 6–8 weeks. The reported success rate in eradicating infection using this treatment protocol is 80–90%.

Chronically infected THRs can be treated with either one- or two-stage revision. The gold standard in most hip revision centres is a two-stage revision. All of the implants and cement must be removed and a thorough ‘membranectomy’ performed. The femoral canal and acetabulum must be reamed until healthy bleeding bone is encountered and the whole surgical field subjected to extensive lavage. A temporary spacer is inserted and this is usually made from antibiotic impregnated cement. If the antibiotic sensitivities are known preoperatively, appropriate antibiotics can be incorporated into the spacer prior to insertion. The wound is closed over a suction drain and the patient mobilized. The same principles apply to the antibiotic regimen as for the acute infections discussed earlier. The reported success rate of two-stage revision in terms of eradicating infection is 90% in most series.

Aseptic loosening is by far the commonest cause of failure of THR and the commonest indication for revision THR. Approximately 3000 revision THRs are carried out each year in the United Kingdom for aseptic loosening. This accounts for 55% of the revision burden for hips.

When a cobalt chrome femoral head articulates with a polyethylene cup wear particles with a diameter of 0.3–10 microns are produced. These particles activate macrophages that, in turn, release cytokines (interleukin (IL)-1α, IL-1β, IL-6), prostaglandins (PGE-2) and tumour necrosis factor (TNFα). These factors not only cause osteolysis but also activate osteoclasts that resorb bone. The result is loosening at the cement–bone interface. Typically a polyethylene acetabular component wears at the rate of 0.1mm per year. Larger head sizes generate greater volumes of wear particles and thin polyethylene components are also more susceptible to wear. It is recommended that the minimum thickness of polyethylene should be 8mm in order to minimize generation of wear particles. Recently, highly cross-linked polyethylene and polyethylene containing vitamin E have been introduced with potential improvements in wear characteristics.

Cementless components can also be subject to aseptic loosening. They normally have a porous coating or are plasma sprayed with hydroxyapatite (HA). The intention is for the component to osseointegrate within the first few months of implantation. In order for osseointegration to occur, the implant must be stable at the time of implantation. Stability is achieved by ‘press-fit’ of the component. Surgical technique must be accurate and it is essential that the correct sized implants are used and inserted in the correct orientation. If primary stability does not occur, the component will move and fibrous ingrowth, rather than osseointegration, will occur, resulting in early loosening of the component. This phenomenon is more common with femoral components than acetabular components and the incidence is around 2% based on the early failure rates quoted in the National Joint Registry.

Loosening may be asymptomatic. Symptomatic patients usually complain of thigh pain. Classically this is ‘start up’ pain experienced first thing in the morning or when getting out of a chair after a period of rest.

Diagnosis is made from the history and plain radiographs in the majority of patients (Figure 7.10.2). It is important to distinguish aseptic from septic loosening, as important management decisions need to be made. In aseptic loosening the CRP and ESR should be normal.

Harris classified loosening into three groups; definitely loose, probably loose, and possibly loose, depending on the radiographic appearances (see Table 7.10.2). Furthermore, the zones of loosening around cemented femoral and acetabular components have been described by Gruen and DeLee (Figure 7.10.3).

The treatment of choice for a non-infected, loose THR is a single-stage revision. The surgical procedure ranges from exchange of head and liner through to revision of both components. This is discussed in more detail in Chapter 7.11.

The reported dislocation rate following THR is 1–3%. The causes of dislocation may be categorized as patient related, surgeon related and implant related. (See Table 7.10.3). The greatest risk of dislocation is within the first 3 months after surgery. Dislocations that occur after the first 6 weeks are more likely to become recurrent.

It is reported that patients with hip dysplasia, rheumatoid arthritis, previous sepsis, those who have had previous hip surgery, and those who have previously suffered a fracture have higher dislocation rates after THR. Patients who lack full motor control of their hips are also at risk, e.g. alcoholics and those who have suffered a cerebrovascular accident. Poor compliance with postoperative rehabilitation and precautions may also result in dislocation.

There are several factors that need to be considered including surgical approach, leg length, femoral offset, soft tissue tension, cup orientation, stem version, impingement, and soft tissue repair. Planning the THR in advance is mandatory. It is imperative that the choice of surgical approach and implants enables the restoration of normal anatomy and function. Traditionally the posterior approach had a higher dislocation rate than anterior or anterolateral approaches. With improvements in surgical technique and, in particular, repair of the posterior capsule the dislocation rates are now similar for all approaches.

Lewinneck described a ‘safe zone’ for the position of the acetabular component. Cups inserted at an inclination of 45 ± 10 degrees and with 15 ± 10 degrees anteversion have greater stability than cups inserted in positions outside this range. Dislocation is more common when cups are inserted more vertically and with incorrect version. When a trial reduction is performed with the definitive acetabular component correctly positioned, the femoral head should be congruently situated within the cup when the patient’s leg is in a neutral position.

It is important to put the hip through a range of motion, particularly internal rotation in flexion and external rotation in extension, to identify potential impingement. Frequently, rim osteophytes need to be trimmed and hypertrophic capsule excised in order to prevent impingement. Finally, the soft tissue tension needs to be considered. If an anterolateral approach is used it is essential that the tendons of gluteus medius and minimus are repaired accurately.

For a hip to stay ‘in joint’ the implants need have a satisfactory head:neck ratio. The smaller the ratio the greater the risk of dislocation and the greater the ratio the less the risk (Figure 7.10.5). There has been a trend in recent years to use larger head sizes in order to prevent this complication. There is a trade off, however, between head size and production of wear particles. With metal-on-polyethylene combinations, larger head sizes, e.g. 32mm or 36mm produce a greater volume of particles, especially with acetabular components less than 8mm thick, and this may lead to early loosening. The wear properties of smaller heads, e.g. 22.225mm as used on the original Charley stem, are better but the dislocation rate is higher. To counteract this problem hard-on-hard bearings (metal-on-metal or ceramic-on-ceramic) are becoming more popular. These allow large head sizes (36mm and above) with a favourable head:neck ratio while avoiding the risk of aseptic loosening.

Key points in preventing dislocation include careful patient selection, thorough preoperative planning, use of appropriate implants, accurate implant positioning, protection of the soft tissues, meticulous wound closure, and well-supervised postoperative rehabilitation.

The usual clinical picture is of a patient who performed a movement at the extreme of range of motion, who felt a ‘clunk’ and then experienced severe pain. The dislocated limb is always short. Posterior dislocations are normally associated with flexion and internal rotation whilst anterior dislocations are usually extended and externally rotated. Plain radiographs confirm the dislocation.

The treatment of choice for a first dislocation is a closed reduction. Ideally the procedure should be carried out in the operating theatre under general anaesthetic. The hip should be screened with the image intensifier to check the orientation of the implants and for impingement.

For recurrent instability, the cause of dislocation needs to be identified. This can be with plain radiographs, screening under image intensification or with computed tomography (CT). The causes of dislocation should be corrected and this almost always requires revision surgery. Sometimes the modular components can simply be exchanged and soft tissue balance improved. Occasionally, extra measures such as constrained cups are required.

The incidence of periprosthetic fracture after THR is rising. Nowadays, in the United Kingdom, the average age of patients undergoing THR is 73 years. Approximately 8% of patients undergoing THR are over 85 years old and about 500 hip revisions are carried out each year for periprosthetic fractures.

These fractures can occur intraoperatively or postoperatively. During surgery fractures are more common when using cementless components. The acetabulum is at risk when it has been deliberately under reamed and the shell is impacted with force in order to achieve ‘press-fit’. The femur is at risk when inserting a ‘press-fit’, tapered implant. The type of fracture ranges from a simple, undisplaced crack in the calcar region to a grossly comminuted, displaced fracture. The femur is much more commonly fractured than the acetabulum and the calcar region should always be inspected after insertion of the femoral component.

Postoperatively, the fracture may follow significant trauma or may be a result of relatively minor trauma combined with a loose implant and/or osteoporosis.

The clinical features are those of an acute fracture of the proximal femur, namely, pain associated with shortening and external rotation of the leg. A thorough neurovascular assessment should be made with particular attention to the foot pulses and peripheral nerves. Plain radiographs should be obtained including an anteroposterior (AP) view of the pelvis and AP and lateral views of the whole femur.

Classification systems are described for intraoperative fractures but they are of limited use and do not guide patient management. The same can be said for postoperative acetabular fractures. In principle, acetabular fractures are classified as type 1 if the component is well fixed or type 2 if the component is unstable. As far as postoperative fractures of the femur are concerned, the Vancouver classification is the most widely used. This classification takes into consideration the location of the fracture, the stability of the implant and the quality of proximal femoral bone stock (see Table 7.10.4). Importantly, this classification system guides the management of the fracture.

Ideally all intraoperative fractures are identified during surgery and fixed appropriately. Stable acetabular fractures may be treated conservatively with a period of protected weight bearing. Unstable fractures need to be reduced and internally fixed. It may be necessary to augment the acetabulum with a cage or support rings to accommodate the cup. Intraoperative femoral fractures are best treated be temporary removal of the implant, reduction of the fracture, fixation with cerclage wires and then reinsertion of the implant.

The management of periprosthetic fractures in the postoperative period is guided by the Vancouver classification and is described in more detail in Chapter 7.12.