Postoperative complications can occur in as much as 9% of patients after primary total knee arthroplasties (TKA), and account for as much as 50% of revision procedures.5 In particular, isolated patellar revision is associated with a high complication rate and recurrent failure.5 Typical complications for the procedure include patellar subluxation, dislocation, fracture, patellar component loosening and fracture during replacement, and patellar clunk syndrome.15,27 These complications probably are attributable to an underlying etiology of patellar maltracking or surgical problems requiring additional treatment.14 Numerous reports of surgical failures have focused on implant design, materials, contact pressure at the knee, wear issues, and surgical techniques such as the amount of bone resection and component position.1,2,5,7,9-11,15,16,22 The cemented all-polyethylene round patellar button is accepted, and improvements in implant design have helped resolve complications.11 Other authors have mentioned the possibility of impingement between the patella and the tibial insert,2,4 but relatively few patellotibial complications have been reported.13
Progress in implant design and operative technique have allowed achievement of deep flexion after TKA.2,3,21 Deep flexion is important in the quality of Japanese patients' lives and in other cultures where sitting directly on the floor with the knees in deep flexion is customary. However, achieving deep flexion can result in the previously mentioned surgical complications, including fracture of the polyethylene tibial post.2,3,8,21
We report the clinical and functional results for five patients with seven deeply-flexed knees in which impingement between the patellae and tibial TKA components occurred.
MATERIALS AND METHODS
We retrospectively reviewed clinical and radiographic data of five patients (with seven knee surgeries) with a mean age of 78 years (range, 73-81 years). There were four women and one man. The diagnoses were osteoarthritis (OA) in four patients (six knees) and rheumatoid arthritis (RA) in one patient (one knee). The mean followup was 50 months (range, 24-73 months) (Table 1).
Cemented posterior-stabilized prostheses were implanted in all knees. Four knees had patellar resurfacing. All surgeries were performed by two surgeons (MK and YN) using standard surgical techniques. The infrapatellar fat pad was left intact except for resecting parts that interrupted the operative field. Patellar buttons were placed in the center of the original patella in the sagittal plane, although some were positioned relatively medial in the axial plane to avoid lateral subluxation.
Patients usually were contacted monthly until 6 months, then at 9 months, 12 months, and yearly thereafter. Some patients were evaluated more frequently. A Hospital for Special Surgery (HSS) score17 was calculated at 6 months and 1 year postoperatively, with a maximum of 100 points. Joint line elevation was measured using the method described by Figgie et al, referring to positions of the tibial tubercle, weightbearing surface of the pre- operative tibial plateau, and postoperative tibial prosthesis on lateral radiographs.12 Changes in the relative position of the joint line preoperatively and postoperatively were defined as the difference between the perpendicular distance from the weightbearing surface of the tibial plateau to the tibial tubercle of the natural tibia, and the perpendicular distance from the weight- bearing surface of the prosthetic tibial component to the tibial tubercle.12 We also calculated Insall-Salvati ratios.18 These data were measured by two independent investigators (H.M. and S.M.).
Flexion of as much as 145° was achieved postoperatively in six knees, and as much as 125° in one knee. The mean joint line elevation was 3.1 mm (range, 0-8 mm) during the operation. The mean Insall-Salvati ratio was 0.89 (range, 0.70-1) immediately after surgery. Four knees had Insall-Salvati ratios less than 1.0. No other serious complications were observed during the early postoperative stage.
All seven knees had impingement between the patella and the tibial component, appearing radiographically at a mean of 25 months postoperatively (range, 1.5-63.5 months). The four knees with patellar resurfacing remained asymptomatic, but the three knees without patellar resurfacing were painful and with reduced range of motion (ROM). The pain eventually resolved almost completely, but the patients still had limited ROM.
The mean Insall-Salvati ratio immediately postoperatively was 0.89 (range, 0.70-1). The Insall-Salvati ratio deteriorated with time to a mean of 0.66 (range, 0.55-0.84). Conversely, the mean HSS score improved from 52 points (range, 23-66 points) preoperatively to 96 points (range, 86-100 points) by 6 months postoperatively.
The joint lines were not excessively elevated (mean, 3.1 mm; range, 0-8 mm) (Table 2), and the tibial components were not overly thick (mean, 10 mm among the four knees with joint line elevation).
An 81-year-old woman with OA had bilateral TKAs using posterior-stabilized prostheses. The patient had a right TKA in January 1995 and a left TKA 1 month later. The femoral prosthesis in the right knee was positioned in a slightly flexed angle. The tibial component had slight overhang above the anterior and posterior edges of the tibial cut surface. The joint line was elevated by 8 mm. The Insall-Salvati ratio immediately postoperatively was 0.70, but decreased to 0.55 by 63.5 months postoperatively when the patellar button began to show mild erosion at the inferior pole. The implants in the left knee were positioned similarly to those in the right knee. The Insall-Salvati ratio in the left knee changed from 0.79 immediately postoperatively, but deteriorated to 0.60 50 months later when the patellar button began to show erosion (Fig 1). No subjective pain or discomfort was reported at 72 months postoperatively, and ROM remained between 0°-145°.
A 73-year-old man with RA had a left TKA using a posterior-stabilized prosthesis in May 1997. When measuring the extension gap using a spacer block, excessive tension resulted in the lateral collateral ligament detaching from the femur. Staples were used for repair. Because of marked bony atrophy we used a long tibial stem. No issues were encountered when placing the prosthesis. The joint line was elevated 5 mm, and the Insall-Salvati ratio changed from 1.0 immediately postoperatively to 0.83 by 9 months later when erosions of the inferior end of the patella were identified on radiographs. Radiographs revealed gradual development of erosion, but knee ROM remained sufficient at 0°-152°, and the patient remained asymptomatic at 73 months postoperatively (Fig 2).
A 76-year-old woman with OA had bilateral TKAs without patellar resurfacing using posterior-stabilized prostheses. The patient had a right TKA in May 1999 and a left TKA 1 month later. The right tibial component was relatively small and was positioned slightly posterior to the anterior edge of the tibial cortex, which was the center of the tibial cut surface. The joint line elevation was 5 mm on the right and 0 mm on the left. The Insall-Salvati ratio immediately postoperatively was 1 on the right and 0.91 on the left. Range of motion was 0°-135° bilaterally. However, ROM in the right knee gradually became limited with concomitant pain on flexion approximately 1.5 months postoperatively. Some osteolytic changes were seen near the inferior pole of the patella, and ROM eventually became restricted to 0°-100°. The Insall-Salvati ratio was 0.77 24 months postoperatively. The left knee had a similar course, with reduced ROM starting approximately 5 months postoperatively and gradually exacerbating to 0°-95° ROM, with an Insall-Salvati ratio of 0.67 (Fig 3). Range of motion remained limited, but pain resolved after conservative treatment. No other problems have been reported.
Numerous complications have been reported after TKAs, but few descriptions of patellotibial impingement have been discussed.2,6,14,15,20,25 Grigoris et al reported two patients with abutment of the patellar button on the tibial stabilizer peg of a stabilized Kinemax® prosthesis.13 Their patients did not have impingement between the patella and the polyethylene tibial component. They suggested impingement might occur from excessive elevation of the joint line, low positioning of the patellar button, or an anatomically low patella, and they thought impingement ultimately was attributable to design flaws in the stabilizing peg and recommended modifying the prosthesis design.13 Argenson et al suggested recessing the anterior margin of the tibial component to reduce extensor mechanism impingement with deep flexion and altering the shape of the patellar components.2 From their report, the problem seemed not to be attributable to just implant design, as impingement needs to be clinically evaluated to reveal the primary causes.
Our case reports have several limitations. A series of five patients is insufficient to provide solid evidence regarding factors predicting impingement or for any new approach, but it does signify importance of avoiding surgically created patella baja. In these five patients, the most important factor contributing to impingement seemed to occur before surgery or with progressive patella baja after surgery. Although not all patients originally had patella baja, and marked progression of this condition occurred in all patients postoperatively. Development of patella baja has been reported as part of infrapatellar contracture syndrome.24 Weale et al reported a decrease greater than 10% in patellar tendon length in 34% of patients who had TKAs.30 This can occur postoperatively as a result of scarring and shortening of the patellar tendon, scarring of the patellar tendon to the anterior tibia, or both.9,24 Whether the infrapatellar fat pad should be removed during TKA is controversial, as resecting the fat pad might facilitate patella baja.24,28 In our patients, patella baja occurred despite retaining the infrapatellar fat pad. Other chemical factors also might facilitate patella baja29 but the precise etiology of patella baja is unclear. We think that TKAs on knees with patella baja before surgery must be done with caution by minimizing soft tissue damage and retaining the infrapatellar fat pad.
An elevated joint line occurred in four knees. Contemporary knee replacement instrumentation elevated the joint line by approximately 2.4 mm (lateral radiograph) to 3.5 mm (AP radiograph) in the Smith and Nephew series using the Genesis II® system described by Kawamura and Bourne.19 In a series of 87 knees, Aglietti et al reported a mean patellar height of 16 mm preoperatively and 4 mm postoperatively, although measurements were made from the anterior edge of the tibial plateau or component.1 In patients with a severe angular deformity requiring extensive ligament release, the tibial component tends to be thicker than the removed bone.10 The joint line then moves proximally and the patella is lowered.10 Figgie et al recommended maintaining the inferior pole of the patellar implant at 10-30 mm proximal to the joint line of the prosthetic plateau, and moving the joint line of the prosthesis 8 mm or less from the natural joint line.12 Joint line elevation can be another cause of acquired patella baja, referred to as pseudo patella baja.9 Four of our patients had moderate pseudo patella baja, with a mean elevation of 3.1 mm. Elevation could cause acquired patella baja by damaging soft tissues with scarring as a result of impingement. Preserving the joint line is important, as this one of few factors that can be managed during the operation.
Numerous factors affect results of TKAs, but achieving deep flexion is one of the most important factors in our patients who had impingement. There is a strong correlation between excellent flexion and patellofemoral contact forces which might cause component failure.26 Flexion of as much as 135° was achieved in six of the seven knees in our patients postoperatively. The knees with marked pre- operative patella baja progressed postoperatively. The possibility of impingement must be considered when deep flexion is achieved in a knee with preoperative patella baja. Continuous impingement results in the accumulation of wear debris and contributes to particle-induced arthritis. Our patients required careful long-term followup.
The risk of patellotibial impingement is not widely recognized, but should be a concern among surgeons intending to achieve more functional ROM after TKA.
We thank Drs. Yasunori Suda, Toshiro Otani, and Shun Kosaka for editorial assistance in the preparation of this manuscript.
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