Patellofemoral Arthroplasty With a Custom-fit Femoral Prosthesis

Abstract

We reviewed the outcomes of a series of patellar arthroplasty operations with custom-fit femoral prostheses to examine the effectiveness of this procedure in relieving pain and restoring function in the knee. Twenty-two patellofemoral arthroplasty operations were performed in 21 patients (mean age, 48.6 years) at 2 institutions between 1994 and 2002. All patients had advanced patellofemoral arthritis and had undergone an average of 2.5 previous patellofemoral operations. The prosthesis, consisting of a custom-fit chrome cobalt trochlear component and an all-polyethylene patellar button, was implanted in a procedure designed to minimize bone resection. Patients later underwent three-view radiography of the knee to confirm that the prosthesis was positioned correctly.

One patient required revision of an undersized patellar button 18 months postoperatively, and 2 other patients had postoperative arthrofibrosis necessitating arthroscopic debridement. No patient required revision of the trochlear component, and no loosening or migration of any component has been found since the first procedure was performed. However, the polyethylene patellar button has worn in 3 patients, and the patella broke in 1 patient.

An average of 60 months postoperatively, patients used the Western Ontario and McMaster Universities Osteoarthritis Index to rate their preoperative and present joint pain, stiffness, and function. Patients’ mean overall ratings (potential range, 24-96) were significantly lower for their present symptoms (28.4) than for their preoperative symptoms (63.4). Mean scores on each subscale also decreased: from 13.0 to 5.5 for pain, from 5.4 to 2.4 for stiffness, and from 45.0 to 20.6 for function. We conclude that, in carefully selected patients, patellofemoral arthroplasty with a custom-fit prosthesis is a viable surgical treatment for isolated patellofemoral arthritis.

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Whereas patellofemoral pain is common in orthopedic patients, isolated patellofemoral arthritis is rare, as well as difficult to treat. The generally accepted initial treatment strategy for this condition is conservative therapy with nonsteroidal anti-inflammatory drugs, physical therapy, and patellar sleeves. However, these measures are often ineffective in patients with advanced patellofemoral arthritis, forcing clinicians to consider surgical intervention.

Various surgical procedures have been developed to address advanced patellofemoral arthritis, but each procedure has significant disadvantages. Patellar elevation, in which the tibial tuberosity is elevated to decompress the patellofemoral compartment, has unpredictable results and is associated with unacceptably high morbidity rates.¹ Patellectomy is also an option but is considered by most to be a salvage procedure; its associated problems include a decrease in quadriceps strength,^2-4 a greater likelihood of poor results when conversion to total knee arthroplasty (TKA) is necessary,^5,6 and poor cosmetic outcomes. Thus, many studies have been published condemning, or at least cautioning against, the use of patellectomy.^7,8 Total knee arthroplasty is effective in many cases,^9-11 but it is highly invasive and requires a lengthy recovery period. Additionally, some evidence suggests that this procedure is not as effective in patients younger than 40 years as it is in older patients.¹²

An alternative approach, patellofemoral arthroplasty, has evolved from isolated patellar resurfacing, to trochlear and patellar replacement, to replacement with custom-fit prostheses. In the present study, we reviewed the outcomes of a series of patellar arthroplasty operations in which patients received a custom-fit femoral prosthesis. Our goal was to examine the effectiveness of this procedure in relieving pain and restoring function in the knee.

Materials and Methods

Patients

Twenty-two patellofemoral arthroplasty operations were performed in 21 patients (13 women, 8 men; mean age, 48.6 years [range, 35-63 years]) at 2 institutions between 1994 and 2002. Each patient gave written informed consent, and the study was approved by the Institutional Review Boards of the University of Texas Health Science Center at Houston and the SUNY Upstate Medical University College of Medicine.

The patients had undergone an average of 2.5 previous operations—all arthroscopic—for their patellofemoral conditions. Selection criteria for patellofemoral arthroplasty included advanced patellofemoral arthritis and no more than minimal maltracking of the patella within the trochlear groove. None of the patients had significant varus or valgus deformities. The most common deformity was lateral tracking/subluxation of the patella. Patellofemoral arthritis was associated with a history of patellar dislocation in 6 cases, osteoarthritis in 12, and posttraumatic arthritis in 4.

Surgical Technique

The prosthesis used consists of a custom-fit chrome cobalt trochlear component and an all-polyethylene patellar button (Custom Performa Knee; Biomet Inc, Warsaw, Indiana). The trochlear component is custom-fabricated by a multistep process. First, a computed tomography (CT) scan is performed on the distal femur. Next, a computer-assisted design program is used to construct a three-dimensional (3-D) model of the distal femur based on the CT image. A 3-mm thick, custom-fit trochlear implant (Figure 1) is then made from cobalt chrome to conform to the 3-D model of the distal femur (Figure 2). Grit is blasted on the interior surface of the implant to improve adhesion.

Figure 1: The trochlear component of the Performa patellofemoral prosthesis. (Reproduced with permission from Biomet Inc.) Figure 2: A three-dimensional model of the distal femur is constructed from a CT image by a computer-assisted design program (A). The cobalt chrome trochlear implant is then manufactured to conform to the model (B).

To implant the device, we used a procedure designed to minimize the amount of bone resection required. First, the knee area was prepped and draped in routine fashion with the use of a pneumatic tourniquet inflated to 350 mm Hg. Then a midline incision was made, starting approximately 5 cm proximal to the patella and running inferiorly to a point just below the tibial tuberosity. Medial and lateral subcutaneous flaps were made, and a lateral retinacular release was performed in which the iliopatellar band was incised from the edge of the vastus lateralis muscle fibers down to the insertion of the patellar tendon into the tibial tuberosity. Then, a medial arthrotomy incision was made, and the patella was everted. A subtotal synovectomy was then performed (Figure 3) with a focus on removing the synovial tissue from the suprapatellar pouch (to allow the trochlear implant to fit snugly to the femur). Additionally, all osteophytes around the trochlea were removed, as was any hypertrophic bone that was found.

Figure 3: The suprapatellar synovium is exposed (A) and removed (B).

After these preparatory steps were completed, the trochlear implant was placed on the anterior aspect of the distal femur (Figure 4); care was taken to assure that the most distal point of the implant was in solid contact with the articular surface. Next, the small amount of articular cartilage remaining under the device was removed with a curette or periosteal elevator. The trochlear component was then attached to the femur with 4 low-profile, 6.5-mm cancellous-bone screws (typically 30 mm in length) (Figure 5).

Figure 4: The custom-fit trochlear implant is placed on the anterior aspect of the distal femur. No cement is required. Figure 5: Low-profile, 6.5-mm cancellous-bone screws are used to affix the trochlear implant to the femur.

Once the trochlear implant was in place, the patellar component of the prosthesis was implanted. In our series, the device used was a standard 2.5-cm–radius polyethylene button similar to those used in TKA. For the implantation of the patellar button, which was intended to cover as much of the patellar surface as possible, the undersurface of the native patella was resected with an oscillating saw. A rectangular stem was fit into an undercut rectangular burr hole on the native patella (Figure 6) and then rotated 90° to lock it into place. In all cases, the patella was reduced onto the trochlea in a fashion that would allow the patellar button to move through the trochlear implant in a 90° arc without the use of sutures. If this degree of movement was not immediately possible, additional release was performed (Figure 7) until the 2 implants could move together perfectly through the 90° arc.

Figure 6: A rectangular burr hole is cut to the size of the stem of the patellar button, and the edges of the hole are undermined (A) to allow the stem (B) to twist and lock into place (C). Figure 7: A lateral release is performed to ensure that the 2 components of the prosthesis can move together in a 90° arc.

Once this final step was completed, a wound drain was placed if necessary, and standard closure was carried out. Patients later underwent three-view radiography of the knee to confirm that the prosthesis was positioned correctly (Figure 8). Postoperative care included standard prophylaxis against deep vein thrombosis and a rehabilitation program that follows the protocol used in patients who have undergone TKA.

Figure 8: Radiographs confirm the correct placement of the trochlear implant on the femur (A, B). Figure 9: Over time, the rectangular burr hole is converted into a circle by the stem of the patellar button. This problem can easily be addressed by reshaping the circular hole into a larger, rectangular one and replacing the patellar button by using a similar twist-lock method.

Data Collection and Analysis

Demographic and medical history data were collected from each patient before the operation. Additionally, the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)¹³ was administered to each patient at follow-up, which was a minimum of 36 months and an average of 60 months postoperatively. The WOMAC consists of 24 items, and patients answer each item on a scale of 1 to 4, with higher ratings indicating worse symptoms. The WOMAC includes three subscales: pain (5 items; range, 5-20), stiffness (2 items; range, 2-8), and function (17 items; range, 17-68). Each patient’s overall WOMAC score is computed as the sum of the patient’s responses to the individual items (range, 24-96).

Patients were asked to respond to each item twice: once to rate their preoperative knee symptoms, and once to rate their present knee symptoms. The questionnaire was administered in the clinic to 19 of the patients and by telephone to the other 2 patients. Mean overall WOMAC scores and scores for each subscale were computed for the preoperative and follow-up questionnaires.

Results

One patient required revision of an undersized patellar button 18 months postoperatively. This patient also required aspiration of effusion 4 times during the follow-up period; all cultures were negative. Two other patients had postoperative arthrofibrosis, which necessitated arthroscopic debridement. No patient required revision of the trochlear component. No loosening or migration of any component has been found since the first procedure was performed. All complications have been related to the polyethylene and the patella: the polyethylene patellar button has worn in 3 patients, and the patella broke in 1 patient.

Patients’ WOMAC ratings of their symptoms at follow-up were significantly lower than their ratings of their preoperative symptoms. The mean aggregate score decreased from 63.4 to 28.4, and mean scores on each subscale decreased as well: from 13.0 to 5.5 for pain, from 5.4 to 2.4 for stiffness, and from 45.0 to 20.6 for function.

Discussion

In our series of 21 patients with patellofemoral compartment disease, patellofemoral arthroplasty with a custom-fit prosthesis was associated with reduced knee pain and stiffness and greater knee function at an average of 60 months postoperatively. These findings are in agreement with those of Sisto and Sarin,¹⁴ whose 25 patients underwent patellofemoral arthroplasty with a different type of custom-fit prosthesis and showed significant improvement in both subjective and objective measures of knee function at a mean of 73 months after surgery.

Our patients’ postoperative WOMAC scores were as good as or better than postoperative WOMAC scores obtained in studies of TKA.^15,16 However, these studies excluded patients who had undergone any previous knee procedures other than arthroscopy, whereas each of the patients in our study had undergone an average of >2 previous knee operations. Also, the patients in the TKA studies were significantly older than ours (approximately 71 years vs 48.6 years). Because TKA may be less effective in patients younger than 40 years than it is in older patients,¹² this procedure could have been less beneficial than patellofemoral arthroplasty to our cohort of patients (although it has also been suggested that patellofemoral arthroplasty is also less effective in younger patients than in the elderly¹⁷).

The procedure used in our series has some important advantages. For one, it causes no loss of bone stock in the distal femur. Bone loss is also minimized during revision, because the custom prosthesis does not require cement to hold it in place. Instead, the trochlear implant is held in place with four screws, and the base of the patellar button can be locked into place by being rotated 90° within the rectangular burr hole cut into the native patella—a technique first described by Smillie in 1962.¹⁸ This rectangular hole is worn into a circular one over time (Figure 9). Should revision become necessary, the hole can easily be reshaped into a larger rectangle and fitted with a new patellar button with a larger base. Like the primary procedure, this revision involves no loss of bone stock.

The patellofemoral arthroplasty procedure described here has two essential features of which surgeons who consider using it should be mindful. First, a lateral retinacular release (which not all surgeons may be comfortable doing) is absolutely necessary to prevent overstuffing and to allow proper tracking. Second, all synovial tissue must be removed from the suprapatellar pouch so that the custom implant will fit properly.

Our study had 2 important limitations that should be acknowledged. First, as in most studies of treatments for rare conditions, our small sample size small—only 22 operations in 21 patients. Second, both preoperative and postoperative WOMAC scores were obtained at the same postoperative time point, 5 years after the procedure, and patients’ recollections of their preoperative pain, stiffness, and function may not have been accurate.

Conclusion

Our findings suggest that, in carefully selected patients, patellofemoral arthroplasty with a custom-fit prosthesis is a viable surgical treatment for isolated patellofemoral arthritis. To confirm this, we are currently collecting follow-up data on approximately 60 additional patients who have undergone this procedure. Also, controlled studies are needed to directly compare the effectiveness of this procedure with that of TKA, particularly in younger patients.

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Authors

Dr Butler is from Houston, Texas, and Dr Shannon is from the Samaritan Orthopedic & Sports Medicine Center, Corvallis, Oregon.

This study was funded in part by Biomet Inc.

Correspondence should be addressed to: James E. Butler, MD, 1225 N Loop W #1055, Houston, TX 77008.