FLEXION | Newsletter for Orthopaedic Surgeons; Volume 2~ Number 3
Supported through an unrestricted educational grant from Howmedica Osteonics
Patella Resurfacing in Total Knee Replacement: Is it an Option?
Edward T. Habermann, MD & Mark Kerner, MD
Total knee replacement is one of the most consistently successful procedures in orthopaedics today, with less than a 5% expected complication rate in primary situations with modern resurfacing techniques. Among these complications, over half are associated with the patella. Resurfacing of the patella has proved to be the most surgeon-dependent aspect of the procedure.
Technology and instrumentation have improved tibial and femoral resurfacing, but few such advances have occurred with patella resurfacing. In response to these complications, many surgeons are reevaluating the need to resurface the patella. At our institutions we have found that resurfacing of the patella is appropriate in nearly all cases. The exceptions are those cases where the patient's anatomic variation, extremely young age, or difficult revision makes successful resurfacing less probable.
The Intact and the Resurfaced Patella
Initially, total knee arthroplasty did not include patella resurfacing. The results of these procedures, while good, had a high incidence of patellofemoral joint symptoms. In 1990, Picetti and colleagues reported on 100 total knee replacements without patella resurfacing: patients with low body weight did well, but at final follow-up, 29% had continued symptoms related to the patellofemoral joint. In response to this large percentage of problems, patella resurfacing is being performed by the majority of orthopaedic surgeons today.
Metal-backed designs had many reports of early failure, mostly secondary to polyethylene wear, but all-polyethylene patellae have shown excellent results.
Resurfacing: Improved Strength, Pain Relief
In 1993, Boyd and colleagues reviewed 891 knees in 684 patients, 396 of which were resurfaced. The complication rate was 4% in the resurfaced group, compared to 12% in the non-resurfaced group. The most prevalent complication in the non-resurfaced group was continuing chronic knee pain. In another study, Enis reviewed 25 cases of bilateral total knee replacements, with resurfacing performed only in the right knees. Resurfacing significantly improved strength and pain relief. In most studies, patients with resurfaced patellae have improved pain relief, improved strength, and lower need for revision secondary to ongoing symptoms.
Patella Resurfacing Complications
The complications associated with patella resurfacing can be described in three categories:
Failure of both the patellar prosthesis and the bone-patella interface is a continuing problem, but one in which significant improvements have been made. Mechanical failure was predominant with metal-backed patella replacements. Thin polyethylene and inadequate fixation of the polyethylene to the metal substrate proved to be the cause of failure in most cases (see Figure 1). Newer, all-polyethylene components with improved geometry have greatly decreased the significance of these mechanical failures (see Figure 2). Poor fixation can continue to be a problem especially in those patients who have undergone overly aggressive bone resections or revisions. Fractures also are related to improper bone resection and to holes made in the patella for fixation. Careful bone resection, made easier with modern instrumentation, may reduce these problems. Osteonecrosis, particularly with extreme soft-tissue releases, has been reported, possibly as a result of isolating the major patella blood supply prior to lateral releases. In our experience, most metal-backed, porous-coated patellar prostheses that required removal for polyethylene wear, maltracking, or subluxation-dislocation have been well-fixed to the bone and are quite difficult to remove.
Figure 1 Metal-backed patellae were prone to failure at the metal-polyethylene interface.
Figure 2 Even with all-polyethylene components, failure secondary to wear is a continuing problems
Inadequate soft-tissue balancing can cause maltracking in both the intact and resurfaced patella. Component orientation, especially rotation, are crucial to patella tracking. With femoral and tibial components properly inserted, careful assessment of tracking without reliance on soft-tissue repair or femoral component geometry is the only way to prevent this complication in both the intact and resurfaced patella.
Post-arthroplasty anterior knee pain is the most common complication encountered when not resurfacing the patella. Preoperative and intraoperative evaluation of the patellofemoral joint is difficult even in the most experienced hands. The presence of anterior knee pain, inflammatory arthropathy, poor tracking as evidenced on preoperative radiographs, and obvious arthritic changes at the time of surgery are clear indications to resurface. Numerous studies of articular cartilage have indicated, however, that dramatic abnormalities in proteoglycan content may exist in articular cartilage before clinical injury is evident. The patella in an affected joint may look quite normal, yet still have pathology that dooms it to failure. Osteonecrosis as a proven cause of persistent anterior knee pain is uncommon.
When Not to Resurface the Patella
In our experience there is a small group of patients who benefit from not resurfacing the patella (see Figure 3). Such patients have significant risk factors for failure of resurfacing. The optimal candidate for total knee arthroplasty without patella resurfacing is a young, lightweight patient with noninflammatory arthritis. At the time of surgery, the patient has a completely normal appearance of patella articular cartilage and demonstrates anatomic patello-femoral tracking. In these young patients, revision secondary to patient longevity can be expected; an intact patella can make that planned revision significantly easier.
Other Indications for Not Resurfacing the Patella Include:
- A revision situation in which adequate mechanical fixation of a patellar component cannot be obtained secondary to bone loss.
- A patellectomy may be appropriate if the patient's level of function will put minimal demands on his knee.
- A patient who has previously suffered a patella fracture in which adequate fixation cannot be obtained.
- The patella with extreme alta or baja, where the tibial and femoral components are in good position and well-fixed. Patients with these parameters often have problematic resurfacing, and may do better if left alone.
The patellofemoral articulation continues to be the most problematic area in total knee arthroplasty. Dynamic instability and mechanical failure can be minimized by careful surgical technique and well-designed prostheses. The efficacy of not resurfacing the patella, however, is generally beyond the ability of the surgeon to evaluate. For most patients, it is unacceptable to undertake a total knee arthroplasty and have the 10-20% probability of significant anterior knee pain that has been found in studies of patients who have not undergone resurfacing. Nonetheless, a patient population does exist in which leaving the patella alone is the best course.
Certainly, further study is needed to refine the indications for patella resurfacing. Little is known about the long-term effect of metal articulation on patellar cartilage. Well-controlled outcome studies looking at the long-term functional and quality-of-life effects of patella resurfacing are needed. Hopefully, with further study, we will be able to more accurately identify those patients who will benefit from not resurfacing the patella.
QUESTIONS AND ANSWERS
What is the best method for cup implantation procedures for an oval-shaped acetabutum?
James Cousins, MD
The dimension of the bony acetabulum that limits the component size is the anterior/posterior measurement. An oval acetabulum usually is larger in the inferior/superior dimension than in the anterior/posterior dimension. If there is sufficient A/P bone stock, enlarging the reaming will allow stable component placement, restoration of the hip center, and satisfactory reconstruction-by using a larger component. This reconstruction method also places the component on host bone, a more durable method. With insufficient A/P bone stock, an oval acetabulum can be converted to a hemisphere by using a structural bone graft. This will restore the hip center to the appropriate level but then relies on bone graft for component stability - a construct with a potentially higher failure rate. Restoration of bone stock may be preferable, however, in young patients or in those patients with mechanical hip centers that are 1.5 to 2 cm higher than anatomic. Also available are custom segmental acetabulum implants that allow biomechanical reconstruction of oval defects. These devices lack long term follow-up, however, and remain investigational.
John Schurman, II, MD
Chief Arthritis & Reconstructive Surgery
Henry Ford Hospital
|Figure 3 In the appropriate patient, an unresurfaced patella can conform well to the prosthesis and provide painfree function. |