Acetabular Revision: Uncemented Hemispheric Components

Introduction

Revision of the acetabular component of a total hip arthroplasty (THA) poses a unique set of challenges for the reconstructive surgeon. A full appreciation of the etiology for the revision will assist in the preoperative planning and execution of an appropriate acetabular reconstruction. There are several etiologies for revision of the acetabular component, including polyethylene wear with or without osteolysis, aseptic loosening, mechanical failure, recurrent dislocation, acetabular component malposition, and septic loosening. The challenges of acetabular revision are related to loss of bone stock, alteration of the hip center, and the need to achieve stability of the prosthesis. The aims of revision are to reconstitute acetabular deficits with the primary goal of recreating the anatomy and biomechanics of the physiologic joint while achieving stable fixation.

Over the past 30 years, a cementless hemispheric component has become the preferred technique for the majority of acetabular revisions. Paramount to revision success is that the implant has initial stability to promote bone ingrowth and remodeling of the acetabulum in order to ensure long-term viability of the arthroplasty. Implant stability is directly related to the characteristics of the acetabular bone deficits, which determines the specifics of the reconstruction process. A standard porous cementless hemispheric shell secured with multiple screws is the implant of choice when there is at least 50% host-bone contact. The introduction of ultraporous metal surfaces with superior bone ingrowth capability has further increased the capabilities of cementless hemispheric acetabular shells.

Indications

The basic principle in the vast majority of arthroplasty procedures is that the physician should operate based on the patient's complaints and not the patient's radiographs. However, in acetabular revision, this is not always the case. Patients presenting with polyethylene wear with or without osteolysis may have varying degrees of symptoms ranging from none to severe incapacitating pain. The physical examination may range from normal to the individual who presents with significant limp and instability with recurrent dislocations. Radiographic evidence of polyethylene wear with significant osteolysis warrants acetabular revision even in the absence of clinical or functional symptoms. Computed tomography (CT) scan evaluation of patients has documented a far greater degree of osteolysis than can be evaluated using standard radiographic techniques.

The imaging studies can be combined with knowledge of the track record of specific components. For example, some older liners are associated with significant wear and osteolysis. The debate in this specific group of patients is whether a complete component revision is necessary or whether there should be consideration for isolated polyethylene exchange with retention of the acetabular component. The requisites for isolated polyethylene exchange alone are an acetabular component that is well fixed, in satisfactory position, has a satisfactory track record for component fixation, and has a satisfactory locking mechanism or adequate size to allow cementation of appropriate polyethylene into the device. However, one of the most common complications of isolated liner exchange is instability, with reported incidence as high as 15% in patients who have undergone the surgical procedure via a posterior approach. The direct lateral approach has reduced this complication. Complete acetabular revision is required in the setting of damage to the locking mechanism or the actual component itself, which precludes the replacement of a new polyethylene liner or the cementation of an appropriate liner into the component. Acetabular revision is also required in cases of severely malpositioned components or a migrated or unstable component that fails stability testing during the operative intervention.

Patients presenting with acetabular component loosening will generally localize pain to the groin, buttock, and anterior medial thigh. Pain is generally associated with weight-bearing activities. Patients frequently describe classic start-up pain, which is severe pain with the first few steps followed by a diminution of the severity of the pain as the patient ambulates. On physical examination, the patient will ambulate with a limp, noting pain in the groin. Pain is generally elicited with the patient supine performing a straight leg raise and, again, localizing the pain to the groin.

Radiographic evaluation generally reveals radiolucent lines at the component-bone interface. These may vary from radiolucencies limited to 1 or 2 of the DeLee and Charnley zones to radiolucencies involving all 3 zones. Radiolucencies in zones I and II are more indicative of loosening versus an isolated radiolucent line in zone III. Serial radiographs are always useful to determine the stability of an acetabular component with respect to cranial and medial migration. Further imaging studies that may be useful are CT scans, which can critically evaluate the bone-prosthetic interface as well as determine the degree of osteolysis and osteopenia surrounding the acetabular component.

Although falling out of favor for use in primary THA, patients with alternate bearing surfaces have unique indications for complete acetabular revision. In patients who have a ceramic-on-ceramic bearing, the incidence of squeaking has been reported to be anywhere from 0.2% to 21%. A variety of etiologies have been described. Several reports indicate that squeaking is component specific. While squeaking itself has not been shown to have any deleterious effects on the arthroplasty, if secondary to component malposition and impingement, the impingement may cause a stress riser on the femoral neck and, ultimately, a fracture of the femoral neck. Additionally, squeaking has been the sole indication for acetabular component revision in patients whose lifestyle is affected by the squeaking itself.

Metal-on-metal THA had a resurgence in the 2000s, with the goal of enhanced stability and decreased wear. However, we soon saw a rise in adverse tissue reaction due to metal debris. Two components were withdrawn from the market ; therefore, patients presenting with these components in place with any symptoms should be counseled on acetabular revision. Symptomatic patients with metal-on-metal articulation should undergo serum metal ion screening. The Medicines and Healthcare products Regulatory Agency (MHRA) in the United Kingdom has recommended for all patients implanted with metal-on-metal devices follow-up at least annually for 5 years and more frequently if symptomatic. In addition, thorough investigation of symptomatic or at-risk patients should include serum ion testing of cobalt and chromium levels, with a second test performed in 3 months in those with levels greater than 7 parts per billion . Cross-sectional imaging studies, including ultrasound or magnetic resonance imaging (MRI), should be performed, and revision considered if imaging reveals soft tissue reactions, fluid collections, or tissue masses. Patients can also be evaluated with a lymphocytic proliferation assay to determine their reactivity to various metals.

It is imperative to rule out periprosthetic joint infection in all patients undergoing total hip revision arthroplasty. A careful history is important and may raise or lower the physician index of suspicion with respect to a septic process. Specifically, did the patient have any wound healing issues following the primary arthroplasty? Was the patient treated with antibiotics for either wound erythema or extremity cellulitis? Are there any constitutional symptoms suggestive of infection, such as fever, chills, or night sweats? Have there been any recent septic processes, such as a dental abscess, upper respiratory infection, or urinary tract infection? Inflammatory markers consisting of a complete blood count (CBC) with differential, erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) should be obtained on all patients. If these inflammatory markers are abnormal, a hip aspiration should be performed not only for culture and sensitivity but also for cell count. Synovial fluid white blood cell (WBC) count greater than 2000/mL with greater than 60% polymorphonuclear leukocytes should raise concern of infection. There are also newer commercially available synovial fluid analysis tests that can evaluate for alpha defensin, synovial CRP, and bacterial-specific DNA. Finally, at the time of surgical intervention, specimens can be sent for pathology testing, looking at the number of white cells per high-powered field. Greater than 5 cells per high-powered field are suggestive of infection and greater than 10 WBC per high-powered field is diagnostic of infection.