The Journal of Arthroplasty Vol. 12 No. 2 1997
Patellar C o m p o n e n t Medialization in Total Knee Arthroplasty A a r o n A. H o f m a n n , M D , T h o m a s K. T k a c h , M D , C h r i s t o p h e r J. E v a n i c h , M D , M a r c e l o P. C a m a r g o , M D , a n d Y o n g d e Z h a n g , M D
Abstract: Intraoperative correction of patellar makracking has traditionally
involved the use of a lateral retinacular release. Problems, however, related to lateral retinacular release include increased postoperative pain and wound healing complications, compromised patellar blood flow, and longer rehabilitation. The purpose of this study was to assess the effect of patellar medialization in total knee arthroplasty. One hundred forty patients underwent total knee arthroplasty using the same components. Two groups of 70 patients each made up the study. Group 1 included patients whose patellar components were centralized on the patella, and group 2 consisted of patients in whom the patellar component was medialized to reproduce the patient's anatomic high point (ie, sagittal ridge). Lateral retinacular release was required in 45.5% of the patients in group 1 compared with 17% in group 2. The technique of patellar medialization is described. Key words: patellar maltracking, patellar medialization, lateral retinacular release, total knee arthroplasty.
Patellar maltracking problems in total knee arthroplasty (TKA) have ranged from 1% to 20%, depending o n the series [1-6]. Multiple reasons for postoperative maltracking have b e e n cited: excessive postoperative valgus, internally rotated tibial or femoral components, malpositioning of tibial or femoral c o m p o n e n t s in the medial/lateral plane, and malpositioning of the patellar compon e n t {7-14]. Intraoperative correction of maltracking has traditionally involved the use of a lateral retinacular release (LRR) {151. Problems related to LRR include increased postoperative pain and w o u n d healing complications, slowed rehabilitation, and compromised patellar blood flow [I6-20]. Some studies have s h o w n that performance of a lateral release does not fully correct the maltracking problem and that, over time, the
patella tends to track laterally again. Thus, it is advantageous to avoid LRR by ensuring p r o p e r patellar tracking achieved by anatomic prosthetic c o m p o n e n t positioning. It has long b e e n the philosophy and aim of the senior surgeon (A. A. H.) that r e p r o d u c t i o n of each patient's natural a n a t o m y and alignment results in the p r o p e r k n e e kinematics and joint balance, thus improving knee range of motion. It seems reasonable that if the a n a t o m y is reproduced o n one side of the patellofemoral joint, it should also be r e p r o d u c e d on the other by placing the high point of a patellar c o m p o n e n t at the same anatomic location as the patient's natural high point (ie, sagittal ridge). The purpose of this study was to f u r t h e r delineate n o r m a l patellar a n a t o m y w i t h respect to the position of the sagittal ridge and to c o m p a r e results of two groups of patients with patellar c o m p o n e n t s placed centrally o n the patella or placed medially so as to r e p r o d u c e the patient's n o r m a l high point.
From the University of Utah Health Science Center, Salt Lake City, Utah. Reprint requests: Aaron A. Hofmann, MD, Department of Orthopedics, 50 North Medical Drive, Salt Lake City, UT 84132.
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Technique of Patellar Component Medialization
Materials and Methods The study population consisted of 140 patients w h o u n d e r w e n t primary TKA. Patients were divided into two consecutive groups of 70 patients each. Group 1 (the control group) consisted of 70 consecutive TKAs that had patellar c o m p o n e n t s routinely placed centrally on the osteotomized patella with no attempt made to medialize the component. Group 2 (the study group) consisted of 70 consecutive TKAs that had patellar c o m p o n e n t s placed medial to the center to reproduce the patient's normal high point. All patients had prim a r y TKAs using the Natural Knee (Intermedics Orthopedics, Austin, TX) p e r f o r m e d by the same surgeon with the same technique except for placem e n t of the patellar component. All patellas placed in the study were cementless metal-backed. More than 1,000 implants were placed by the senior a u t h o r prior to the study, which should have eliminated any learning curve o n the instruments or implant. The study population consisted of 60 m e n and 80 w o m e n . Demographics for each group were similar (Table 1). All patients were evaluated radiographically before and atter surgery using 45 ° Merchant views. Measurements of each patella were made from these radiographs using a digitizing tablet and a computerized program (Research Metrics, Orthographics, Salt Lake City, UT) that corrects for radiographic magnification. This computerized program has been found to be accurate to 0.001 mm. Patellar width and location of the anatomic high point (sagittal ridge) from the center of the patella were recorded. In addition, patellar tilt and translation were measured. Patients were evaluated clinically before surgery and at 1 year afterward using the modified 100-point Hospital for Special Surgery knee score, w h e r e 40 points are given for absence of pain, 25 points for stability, 25 points for range of motion, and l0 points for strength (Table 2).
Following preparation of the femoral and tibial surfaces, the patella is everted and grasped with a towel clamp t h r o u g h the synovial covering at the quadriceps tendon. The synovial reflection a r o u n d the patella is released using electrocautery to the level of the quadriceps t e n d o n and patellar ligament. Initially, a caliper is used to measure the patellar thickness so that this can be reproduced following implantation of the patellar c o m p o n e n t . The sagittal ridge is t h e n identified and electrocautery is used to mark a spot on the center of the sagittal ridge at the midpoint b e t w e e n the superior and inferior portions of the articular surface (Fig. 1). In cases of severe arthrosis, this m a y be an approximation of the normal ridge. A ~-inch drill is t h e n used to drill into the patella at this m a r k e d position to a depth of 10-mm, the first mark on the calibrated drill bit (Fig. 2). The o s t e o t o m y guide is t h e n placed at the medial and lateral osteocondylar junction to avoid wedge cutting of the patella. The cut is made removing 7 m m of bone. The previously drilled hole is t h e n identified using a h e m o stat. The patellar sizer is next used to identify the correct size of patellar c o m p o n e n t that will allow the center of the patellar c o m p o n e n t to be placed directly over the drill hole so as to reproduce the position at the patient's original high point while still allowing for a continuous rim of bone a r o u n d the c o m p o n e n t (Fig. 3). The correct-size patellar r e a m e r is t h e n selected and placed in a position so that the drill hole is in the center of the r e a m e r bushing (Fig. 4). The r e a m e r is used to r e m o v e an additional 3 m m of bone to allow for countersinking a 10 m m patella. The drill guide is t h e n placed and the appropriate drill holes for the pegs are made to accept a metal-backed or all-polyethylene patellar c o m p o n e n t . Once all c o m p o n e n t s have been implanted, assessment of patellar tracking is
T a b l e 1. P a t i e n t D e m o g r a p h i c s
No. of patients Average patient age Preoperative diagnosis Osteoarthritis Rheumatoid arthritis Avascular necrosis Trauma Hemophilia Lupus
Group 1 (Patella Centralized)
Group 2 (Patella Medialized)
70 71.9 y
70 68.5 y
62 7
57 7
i
I
3 1 I
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Table 2. Modified Hospital for Special Surgery Knee Scoring System Descriptor
Points
Pain Rest pain C o n t i n u o u s : b e d - c h a i r status, narcotics I n t e r m i t t e n t : occasional narcotics After excessive activity Support r e q u i r e d because of p a i n Double support only Limits excessive w a l k i n g Single support only No support r e q u i r e d due to p a i n Limits r o u t i n e w a l k i n g Limits excessive w a l k i n g W e a t h e r - r e I a t e d ache, starting discomfort No p a i n
0 2 5 l0 15 20 25 30 35 40
Stability ( m e a s u r e d as total v a r u s - v a l g u s are, extension) 0o-6 ° 70-9 ° 10°-12 ° 13°-15 ° 16°-18 ° 19°-21 ° >21 °
25 21 17 13 9 5 0
F i g . 1. M a r k i n g m i d p o i n t of patellar sagittal ridge (ie, h i g h point).
M o t i o n ( m e a s u r e d as total passive are) 0°-10 ° 11°-30 ° 31°-50 ° 51°-70 ° 71°-90 ° >90 °
0 4 10 15 20 25
Q u a d r i c e p s s t r e n g t h ( m e a s u r e d as % of n o r m a l for age a n d gender) >75 %, c a n n o t b r e a k quadriceps p o w e r 50 % - 7 4 % , can b r e a k quadriceps p o w e r <50%, h a r d to m o v e t h r o u g h arc of m o t i o n
I0 5 0
Subtractions E x t e n s i o n lag, flexion c o n t r a c t u r e I°-15 ° 16°-30 ° 31 °-45° >45 ° Fixed varus (-) valgus (+) deformity (normal, +3 ° to +8 ° ) Varus +2 ° to -2 ° -3 ° to - 7 ° >-7 ° Valgus +2 ° to +13 ° +14 ° to +18 ° >+18 °
-4 -8 -12 -16
F i g . 2. Drilling patellar h i g h p o i n t to d e p t h of 10 m m .
-4 -8 -12 -2 -4 -6
(From E v a n i c h et a1.,22 w i t h permission.)
m a d e using the n o - t h u m b s t e c h n i q u e by placing the k n e e t h r o u g h a full range of motion. If the patellar c o m p o n e n t tends to lift off on the medial edge or track laterally, an outside-to-inside LRR is p e r f o r m e d w i t h the k n e e flexed to 90 °, taking care to preserve the superior lateral geniculate artery. Postoperative sunrise views of the patella confirm a n a t o m i c p l a c e m e n t of the c o m p o n e n t (Fig. 5).
F i g . 3. D e t e r m i n i n g appropriate-size patellar c o m p o n e n t . Patellar sizing guide c e n t e r e d over drill hole.
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Clinical evaluation at least 1 year after surgery showed no differences with average modified Hospital for Special Surgery knee scores of 98 points for each group. Complications were similar for both groups with no infections, no patellar fractures, and no deep vein thrombosis seen. One patient in group 1 had a subluxating patella, whereas all patients in group 2 had patellas that tracked normally. Postoperative closed knee manipulations were performed in a total of 25 patients for flexion less t h a n 90 ° at 6 to 8 weeks after surgery. These manipulations were p e r f o r m e d in 14% of knees in group l compared with 3 % in group 2. Fig. 4. Appropriate-size patellar reamer centered over drill hole.
Discussion Results Anatomically, the sagittal ridge (ie, high point) showed significant variability in its position with respect to the center of the patella. Measuring from the midpoint of the patella, the location of the sagittal ridge ranged from 0.54 to 11.6 m m medial to the patellar midpoint. The high point was found medial to the patellar midpoint in every case. The average distance medial to the midpatella was 5.46 m m (range, 0.59-11.67 mm) in group I and 5.48 m m (range, 0.54-10.66 ram) in group 2, with an overall average of 5.4 mm. After surgery, the high point of the patellar c o m p o n e n t averaged 0.94 m m (range, 0.17-3.51 mm) medial to the midpatella in group 1 and 2.80 m m (range, 0.00-6.07 mm) in group 2 (Table 3). The m e a n postoperative patellar tilt at the most recent follow-up visit was 6.1 °, with m e a n patellar translation of 5.9 m m in Group 1 compared with 4.5 ° patellar tilt and 3.6 m m patellar translation for group 2. A LRR was required in 45.5% of cases in group 1 and only 17% in group 2, showing a significant difference (P < .05) b e t w e e n the two groups.
Fig. 5. Postoperative medialized patella.
Patellar tracking problems remain significant in TKA. It is one of the most frequent reasons for revision surgery. Reported incidences have ranged from less than 1% to as high as 20% [1-6]. Subluxation is m u c h more c o m m o n than frank dislocation of the patella [7]. Patellar maltracking after TKA is a multifactorial problem. Factors affecting the tracking of the patella include c o m p o n e n t design, c o m p o n e n t malalignm e n t resulting from excessive valgus, internal rotation of the tibial or femoral components, lateral placement of the patellar component, underresection of the patella or oversizing the femoral component, soft tissue imbalance, and trauma. Any alteration in knee kinematics that increases the patient,s Q-angle or tightens the lateral retinaculum increases the risk of maltracking [7-14]. It is important to closely assess patellar tracking at the initial arthroplasty, as results of revision surgery for patella-specific problems have been variable and involve significant complications including continued subluxation, dislocation, patellar fracture, tibial tubercle avulsion, and infection [7,10,11 ]. This study was u n d e r t a k e n to assess only one factor involving patellar tracking, patellar compon e n t placement. Consecutive TKAs performed by the same surgeon using the same technique and knee prosthesis were used for this study. The only alteration made in technique was a change in positioning of the patellar c o m p o n e n t . Using r a n d o m ized groups rather t h a n consecutive groups could have reduced the possibility of experimental bias. The n o r m a l patellar articular surface a n a t o m y reveals that the larger more convex lateral facet is separated from the smaller more concave medial facet by a sagittal ridge. The literature is very sparse with respect to the variability of n o r m a l patellar
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Table 3. Results Group 1 (Patella Centralized) Average distance from center of patella to Sagittal ridge (preoperative) Center patellar component (postoperative) Lateral retinacular release Closed manipulation-[Average Hospital for Special Surgery total knee score (1 year after surgery)
Group 2 (Patella Medialized)
5.46 m m (0.59-11.67)* 0.94 mm (0.17-3.51) 45.5% 14% 98
5.48 m m (0.54-10.66) 2.80 m m (0.00-6.07) 17% 3% 98
*Ranges in parentheses, tClosed manipulation performed 6 to 8 weeks after surgery for knee flexion less than 90 °,
anatomy. Briard and Hungerford have reported average patellar dimensions as 2.5 cm thick and 3 m m wide and the sagittal ridge being located 14 m m from the medial margin a n d 23 m m from the lateral margin [9]. Pace et al., using the c o m p u t e r digitizing program, have studied the position of the patellar sagittal ridge [15]. Their m e a s u r e m e n t s have revealed that the average patellar thickness is 24 m m for m e n and 22 m m for w o m e n and the average width is 44.9 m m with a wide range (32-57 mm). In that study, the position of the sagittal ridge was located an average of 4.6 m m medial to the midpoint (range, 2.3-8.3 m m media). These results are similar to the values found in our current study w i t h the sagittal ridge located an average of 5.47 m m medial to the midpoint of the patella (range, 0.59-11.67 m m medial to the midpoint). It is interesting to note that no patella had the sagittal ridge lateral to the midpoint. No clinical difference was seen b e t w e e n the two groups at 1 year with respect to range of m o t i o n and knee scores. The most striking statistically significant (P < 0.05) difference seen b e t w e e n the two groups is the incidence of LRR. The group with centrally placed patellas required lateral release for patellar maltracking in 45.5% of the cases. Lateral release was required in only 17% of cases in the group of patients with patellar components medialized e n o u g h to reproduce the anatomic high point. This m a y suggest that centralized patellas can be made to track adequately with use of a lateral release, but it is our opinion that performance of a LRR is not a benign procedure. This procedure tends to increase postoperative pain and swelling and slows the rehabilitation process. It has also b e e n s h o w n to decrease patellar blood flow and m a y increase the risk of patellar fracture [16-19]. In one study, L R R was seen to increase w o u n d healing problems and infection rates [20]. It seems logical to assume that lateral retinacular release should be avoided if possible.
There was also a difference (P = 0.092) in the incidence of need for postoperative manipulation. F o u r t e e n percent in the centralized group and 3% in the medialized group required manipulation for decreased knee flexion. Our criterion for d o s e d manipulation is knee flexion less t h a n 90 ° at 6 to 8 weeks after surgery. We believe that p o o r postoperative m o t i o n is very closely related to swelling and arthrofibrosis. The increased incidence of postoperative swelling and arthrofibrosis m a y be related to the increased incidence of LRR in Group 1. Although the average postoperative difference b e t w e e n the two groups with respect to positioning of the patellar c o m p o n e n t s was only 2 ram, the range was variable as in the normal anatomy. There was only a slight difference b e t w e e n the groups at 1 year with respect to patellar translation and tilt; however, we do not expect these differences to change significantly with time as Bindelglass et al. had reported no change in patellar position after 3 m o n t h s [21]. We believe that medialization of the patellar c o m p o n e n t can make a significant difference in joint kinematics.
Conclusion The a n a t o m y of the n o r m a l patella is such that the sagittal ridge is placed asymmetrically with wide variability but always toward the medial aspect of the patella. The average medialization is 5.47 mm, which m a y be helpful in revision situations w h e r e the anatomic high point is not k n o w n . W h e n placing a symmetric patellar component, it is important to attempt to place the high point of the c o m p o n e n t at the location of the individual patient's n o r m a l high point to reproduce patellar tracking. This will result in the n e e d for fewer ERRs and possibly reduce the n e e d for closed manipulations. It m a y also decrease patellar tracking problems and result in improved survivorship of the patellar c o m p o n e n t .
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