THE ASSOCIATION BETWEEN VARUS VALGUS ALIGNMENT AND PATELLOFEMORAL OSTEOARTHRITIS

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1 1874 ARTHRITIS & RHEUMATISM Vol. 43, No. 8, August 2000, pp , American College of Rheumatology THE ASSOCIATION BETWEEN VARUS VALGUS ALIGNMENT AND PATELLOFEMORAL OSTEOARTHRITIS SADAF ELAHI, SEPTEMBER CAHUE, DAVID T. FELSON, LASZLO ENGELMAN, and LEENA SHARMA Objective. Little is known about risk factors for patellofemoral (PF) osteoarthritis (OA). The lateral vector at the PF joint increases the likelihood of lateral PF versus medial PF pathology. Both valgus and varus malalignments affect forces at the PF joint and may predispose to PF OA. We examined whether lateral PF OA is more common than medial PF OA, whether valgus malalignment is more frequent in lateral PF OA than in medial PF OA, and whether knees with PF OA are more often valgus than knees with isolated tibiofemoral (TF) OA. Methods. In 292 knee OA patients, we obtained semiflexed, fluoroscopy-confirmed radiographs of the TF joint and weight-bearing, 30 flexion, axial views of the PF joint. Varus valgus alignment (the angle formed by the intersection of the mechanical axes of the femur and tibia) was measured on a full-limb radiograph. Results. Lateral PF OA was more common than medial PF OA (P < ). Forty-three of 75 knees with lateral PF OA had valgus malalignment compared with only 5 of 21 patients with medial PF OA (P ). Conversely, varus malalignment was more likely in the medial PF OA group. Knees with isolated PF OA were more likely to have valgus malalignment than those with isolated TF OA (P ), as were knees with mixed PF TF OA (P ). Conclusion. Varus valgus alignment may influence the risk of PF OA and, in particular, which compartment is affected. Osteoarthritis (OA) of the knee is a leading cause of chronic pain and disability (1,2). The knee is a Supported by NIH grant AR Sadaf Elahi, MD, September Cahue, BS, Laszlo Engelman, BS, Leena Sharma, MD: Northwestern University, Chicago, Illinois; David T. Felson, MD, MPH: Boston University, Boston, Massachusetts. Address reprint requests to Leena Sharma, MD, Division of Rheumatology, Northwestern University, Ward Building 3-315, 303 East Chicago Avenue, Chicago, IL Submitted for publication January 19, 2000; accepted in revised form April 7, complex, tricompartmental synovial joint comprised of the medial tibiofemoral (TF), lateral TF, and patellofemoral (PF) compartments. PF involvement in OA has been linked to greater disability (3). However, natural history studies continue to emphasize the TF compartments. OA development or progression in the PF compartment may be influenced by a profile of factors different from those in either medial or lateral TF disease. Also, the rate of OA progression may differ between compartments. Both biomechanical and clinical studies provide support for these theories. First, the designs of the PF and TF compartments each reflect unique functions and mechanics (4). The same activity or local factor does not have the same effect on the PF and TF articular cartilage. Second, patients with knee OA may have disease isolated to the PF compartment, isolated to one TF compartment, or concomitant in the PF compartment and one TF compartment (5). In patients with PF OA and TF OA in the same knee, no correlation was found between OA severity in the PF and TF compartments (6). Among its functions, the patella lengthens the lever arm of the quadriceps muscle in effecting extension or resisting flexion throughout the knee s range of motion, allows a wider distribution of compressive stress on the femur, acts as a guide for the quadriceps tendon in centralizing the input from the 4 aspects of the quadriceps muscle, controls the capsular tension of the knee, and protects trochlear and condylar cartilage by acting as a bony shield (4,7). The relatively lateral position of the tibial tubercle in full knee extension produces the Q-angle (i.e., the angle formed by the intersection of the line of application of the quadriceps force with the center line of the patellar tendon) (8). The Q-angle adds a strong, laterally directed component to the contact force (9). With contraction, the point of origin of the quadriceps at the proximal femur and the point of insertion at the tibial tubercle move closer together. Because this action occurs through the patella,

2 VARUS-VALGUS ALIGNMENT AND PF OA 1875 which is more medially placed than these 2 points, a bowstring effect draws the patella laterally. The lateral vector is resisted by the vastus medialis, the medial retinacular structures, and the lateral facet of the trochlea (8). It is likely that the lateral vector contributes to the predominance of pathology on the lateral side of the PF joint, including dislocations, lateral pressure syndromes, and possibly, PF OA (8). At physiologic Q-angles, articular pressure is distributed evenly on the lateral and medial patellar facets (9). Either an increase or a decrease in Q-angle may have adverse effects and, in theory, predispose to PF OA. Huberti and Hayes found that an in vitro increase in Q-angle resulted in 2 types of contact-pattern change: a lateral shift of the contact area with unloading of the medial facet, which occurred in 50% of knees, or a separation of contact areas into distinct lateral and medial regions (9). A decrease in Q-angle resulted either in a medial shift of the contact area, with unloading of the lateral facet, or in a separation of contact areas into lateral and medial regions. Direct, longitudinal evidence of a relationship between the Q-angle and the development of PF OA is not available. Coronal TF alignment is a determinant of the Q-angle and patellar tracking. Valgus malalignment leads to an increased Q-angle and an increase in the force on the lateral patellar facet. Varus malalignment leads to a decreased Q-angle and an increase in the force on the medial patellar facet. The theory that varus valgus alignment may contribute to the development of PF pathology has been described (7,10). However, the relationship between malalignment and PF OA has been minimally examined. If valgus malalignment is a risk factor for lateral PF OA, a higher prevalence of valgus knees would be expected among those with lateral PF OA than among those with medial PF OA. Correspondingly, if varus malalignment is a risk factor for medial PF OA, a higher prevalence of varus knees would be expected among those with medial PF OA. If lateral PF OA is more common than medial PF OA, and if valgus malalignment is a risk factor, then knees with isolated PF OA would be more likely to be valgus than knees with TF OA. In this study of knee OA patients, we examined whether lateral PF OA was more common than medial PF OA, whether valgus malalignment was more frequent in lateral PF OA than in medial PF OA, and whether knees with PF OA were more often valgus than knees with isolated TF OA. PATIENTS AND METHODS Patients. Three hundred patients with knee OA were enrolled in the baseline phase of a longitudinal study examining the contribution of mechanical, neural, and muscular factors to radiographic progression and functional decline in knee OA. Patients were recruited from the community through advertising in 67 neighborhood organizations and senior centers, as well as in local periodicals, press releases, letters to patients listed in the Aging Research Registry of the Buehler Center on Aging of Northwestern University, and referrals from local physicians. Inclusion and exclusion criteria were based on consensus recommendations for knee OA studies (11,12). Inclusion criteria were definite osteophyte presence in the medial and/or lateral TF compartment (i.e., Kellgren/Lawrence radiographic grade 2) of one or both knees and at least a little difficulty with knee-requiring activities. Exclusion criteria were intraarticular corticosteroid injection into either knee within the previous 3 months, uncomplicated knee surgery within the previous 6 months, complicated knee surgery within the previous year, bilateral total knee replacement, history of avascular necrosis, rheumatoid arthritis, or any other systemic inflammatory arthritis, periarticular fracture, Paget s disease, villonodular synovitis, joint infection, ochronosis, neuropathic arthropathy, acromegaly, hemochromatosis, Wilson s disease, osteochondromatosis, gout, recurrent pseudogout, or osteopetrosis. Radiographic assessment of TF OA and PF OA. The protocols used were those recommended by the OA Research Society International (OARSI) for studies of knee OA (13). These protocols include knee position, criteria for x-ray beam alignment relative to the center of the knee, use of radioopaque markers to account for radiographic magnification, and definition of anatomic landmarks for measurement. To examine the TF compartments, the anteroposterior view was obtained in the weight-bearing, semiflexed position to superimpose the anterior and posterior joint margins. The heel was fixed and the foot rotated until the tibial spines were central relative to the femoral notch. Knee position was confirmed by fluoroscopy. To visualize the PF compartment, 30 flexion, axial (i.e., skyline) views of the PF joint were obtained in the weight-bearing position. In this protocol, the tube is positioned so that the x-ray beam is directed vertically downward, with a standard film-to-focus distance. The knee to be examined is flexed to 30 from the vertical, and the leg is positioned so that it is aligned in the vertical plane. The radiographic plate is placed on a step positioned below the knee. The central ray of the x-ray beam is directed to project through the PF joint space (for the complete protocol, see ref. 13). OARSI atlas grades and representations (13) were used for all radiographic assessments. To assess PF joint space narrowing, a 4-grade scale (0 none, 1 mild/possible, 2 moderate/definite, and 3 severe) was used. The lateral and medial PF compartments were each graded separately. The presence of PF osteophytes was defined as a grade of definite or greater on a similar 4-grade scale for at least 1 of 4 PF surfaces (i.e., lateral patellar, lateral trochlear, medial patellar, and medial trochlear). TF joint space narrowing was assessed separately for medial and lateral TF compartments using a

3 1876 ELAHI ET AL similar 4-grade scale; the highest grade (i.e., for the most severely narrowed joint space) was considered in analysis. TF osteophytes were graded as none, mild, definite/small, moderate, or severe. Each of 4 TF surfaces was graded separately (i.e., medial tibial, medial femoral, lateral tibial, and lateral femoral), and the highest grade per knee was considered in analysis. All radiographs were obtained in the same unit by 1 of 2 technicians trained in these protocols. One experienced reader performed radiographic assessments using an atlas (13). For this reader, the reliability for the categorical scales, using kappa coefficients, was good (e.g., ). Measurement of alignment. To assess alignment, a single, anteroposterior radiograph of the entire lower extremity was obtained. A inch graduated grid cassette was used as recommended (14). By filtering the x-ray beam in a graduated manner, this cassette accounts for the unique soft tissue characteristics of the hip and ankle, so that all landmarks in the lower extremity can be visualized well. The size of the cassette allowed inclusion of the full lower extremity of taller patients. The patient stood without footwear, positioned so that the tibial tubercles faced forward. As a site adjacent to the knee not distorted by the presence of OA, the tibial tubercle was used as the positioning landmark (15). The patella is often used for this purpose in studies of normal knees (14), but could not be used in the current study given the potential presence of PF subluxation or OA. The x-ray beam was centered at the knee at a distance of 8 feet. Settings of ma-seconds and kv were used, depending on limb size and tissue characteristics, to ensure landmark visualization. Alignment was measured on the full-limb radiograph as the angle formed by the intersection of the mechanical axes of the femur and the tibia, as recommended (15 17). Specifically, the angle measured was formed by the intersection of the line from the center of the femoral head to the center of the femoral intercondylar notch with the line from the center of the ankle talus to the center of the tips of the tibial spines. A knee was defined as being varus when alignment (the intersection of the mechanical axes of the femur and the tibia) was 0 in the varus direction, valgus when alignment was 0 in the valgus direction, and neutral when alignment was 0, as done previously (14,17 19). The angle made by the intersection of lines drawn through the femoral and tibial shafts on a knee radiograph was not used in the current study, since it does not account for variation in the proximal femur, femoral shaft, tibial shaft, or ankle (20), it varies greatly among normal subjects in comparison with full-limb measurements (17), and it is not typically used in orthopedic clinical or biomechanical studies. All measurements of alignment were made by a single experienced reader. For this reader, reliability was high for measurements of both varus (intraclass correlation coefficient [ICC; model 1,1] 0.99) and valgus (ICC 0.98) alignment. Measurement of body mass index (BMI) and leg dominance. BMI was measured as the weight (kg)/height (m 2 ). Leg dominance was ascertained using the question In order to kick a ball, which leg would you use? Statistical analysis. The dominant knee of each patient was used in analysis, to account for variation in how patients load dominant versus nondominant knees. The proportion of dominant knees in which PF narrowing was more Table 1. Characteristics of 292 patients with knee OA* Full cohort (n 292) Isolated PF OA (n 32) Isolated TF OA (n 92) Age, mean SD years Women, no. (%) 208 (71) 28 (80) 61 (66) BMI, mean SD weight (kg)/height (m 2 ) *OA osteoarthritis; PF patellofemoral; TF tibiofemoral; BMI body mass index. Patients with isolated moderate-to-severe PF OA had definite PF narrowing and osteophytes, no TF narrowing, and no or small TF osteophytes in the dominant knee. Patients with isolated moderate-to-severe TF OA had definite TF narrowing and osteophytes, no PF narrowing, and no PF osteophytes in the dominant knee. severe in a single PF compartment was determined. The Pearson chi-square test was performed to determine whether valgus malalignment was more frequent in lateral PF OA than in medial PF OA, whether knees with isolated PF OA were more often valgus than knees with isolated TF OA, and whether knees with mixed PF-TF OA were more often valgus than knees with isolated TF OA. RESULTS Of the 300 patients with knee OA enrolled in the study, 8 were excluded since they had undergone total knee replacement in their dominant knees. No patients had undergone osteotomy or meniscectomy in their disease management. Sample characteristics of the full cohort are shown in Table 1. Among patients with varus knees, the mean SD varus malalignment was In patients with valgus knees, the mean SD valgus malalignment was Ninety percent of the sample was right-leg dominant. Frequency of lateral PF OA versus medial PF OA. Of patients with 292 dominant knees, 96 had evidence of asymmetric PF joint space narrowing (i.e., 1 compartment more severely narrowed than the other). Of these 96 patients, 75 had evidence of greater lateral PF narrowing, and 21 had evidence of greater medial PF narrowing (P ). PF compartment narrowing may reflect subluxation, subluxation with OA, or OA alone. Therefore, we also examined knees with other evidence of PF OA, as shown by definite PF osteophyte presence coupled with asymmetric PF compartment narrowing. Sixty-seven patients had evidence of lateral PF OA, and 19 had evidence of medial PF OA (P ). There were no

4 VARUS-VALGUS ALIGNMENT AND PF OA 1877 Figure 1. Varus valgus alignment in patients with lateral patellofemoral (PF) versus medial PF narrowing. Shown are the numbers of subjects with valgus or varus malalignment within lateral PF narrowing or medial PF narrowing groups. Valgus malalignment was more common in those with lateral PF narrowing, and varus malalignment was more common in those with medial PF narrowing. OA osteoarthritis. differences in age, sex, or BMI between patients with lateral and medial PF narrowing or between patients with lateral and medial PF OA. Frequency of valgus malalignment in lateral PF OA versus medial PF OA. Valgus malalignment was present in 43 of 75 patients (57%) with lateral PF narrowing and in only 5 of 21 patients (24%) with medial PF narrowing (Figure 1). Patients with lateral PF narrowing were more likely to have valgus malalignment than those with medial PF narrowing, while patients with medial PF narrowing were more likely to have varus malalignment than those with lateral PF narrowing (P ). Results were similar when knees with other evidence of PF OA were examined (Figure 2). Valgus malalignment was present in 38 of 67 patients (57%) with lateral PF OA and in only 5 of 19 patients (26%) with medial PF OA. Likelihood of valgus malalignment in knees with isolated PF OA versus knees with isolated TF OA. Thirty-two of 292 patients (11%) had isolated moderateto-severe PF OA (definite PF narrowing and osteophytes, no TF narrowing, and no or small TF osteophytes) in the dominant knee, while 92 of 292 patients (32%) had isolated moderate-to-severe TF OA (definite TF narrowing and osteophytes, no PF narrowing, and no PF osteophytes) in the dominant knee. Age, sex, and BMI for these PF and TF groups are shown in Table 1. Valgus malalignment was present in 20 of 32 patients (63%) with isolated moderate-to-severe PF OA and in 24 of 92 patients (26%) with isolated moderate-to-severe TF OA (Figure 3). Patients in the PF group were more likely to have valgus malalignment than those in the TF group (P ). The relationship between valgus malalignment and PF OA persisted after accounting for sex. Likelihood of valgus malalignment in knees with mixed PF-TF OA versus knees with isolated TF OA. Forty-nine of 292 patients (17%) had mixed disease in the dominant knee (definite PF narrowing and osteophytes and definite TF narrowing and osteophytes), while 92 of 292 patients (32%) had isolated moderateto-severe TF OA in the dominant knee. Valgus mal- Figure 2. Varus valgus alignment in patients with lateral PF OA versus medial PF OA. Shown are the numbers of subjects with valgus or varus malalignment within lateral PF OA or medial PF OA groups. Valgus malalignment was more common in those with lateral PF OA, and varus malalignment was more common in those with medial PF OA. See Figure 1 for definitions.

5 1878 ELAHI ET AL Figure 3. Varus valgus alignment in patients with isolated moderateto-severe PF OA, mixed PF-tibiofemoral (TF) OA, and isolated moderate-to-severe TF OA. Shown are the numbers of subjects with valgus or varus malalignment within each of the 3 groups. See Figure 1 for other definitions. alignment was present in 27 of 49 patients (55%) with mixed disease and in 24 of 92 patients (26%) with isolated moderate-to-severe TF OA (Figure 3). Patients with mixed disease were more likely to have valgus malalignment than those in the TF group (P ). DISCUSSION In summary, lateral PF OA was more common than medial PF OA. Valgus malalignment was more likely in those with lateral PF OA, and varus malalignment was more likely in those with medial PF OA. Knees with PF OA were more often valgus than knees with isolated TF OA. These results suggest that varus valgus alignment may influence the risk of developing PF OA and, in particular, which PF compartment is affected. The finding that lateral compartment involvement was more common than medial compartment involvement is consistent with findings of previous clinical studies of PF OA (6). We found valgus malalignment more frequently in knees with lateral PF OA, regardless of whether the definition of PF pathology required only lateral joint space narrowing or both PF osteophyte presence and lateral narrowing. These results are consistent with the expected impact of an increased Q-angle and lateral vector. This crosssectional study cannot reveal the direction of the relationship between varus valgus alignment and PF OA. Biomechanical studies suggest a mechanism by which varus valgus alignment may contribute to PF OA. However, it remains possible that another factor stresses both lateral PF and lateral TF compartments, and that the greater prevalence of valgus malalignment is secondary to lateral TF OA. Therefore, we examined the lateral PF OA group and found that roughly equal thirds had evidence of medial TF OA, lateral TF OA, and no TF narrowing, reducing the likelihood that valgus malalignment is the consequence of lateral TF OA. It is more difficult to envision a route by which lateral PF OA itself might lead to valgus malalignment. We also found that knees with PF OA were more often valgus than knees with TF OA. Because medial TF OA may cause varus malalignment, the finding that valgus malalignment was more frequent in PF OA than in TF OA might only reflect the absence of TF OA as an inducer of varus malalignment in the isolated PF group. To address this possibility, we also compared knees with mixed PF TF OA to knees with isolated TF OA, and found that knees were more likely to be valgus when PF OA coexisted with TF OA. The biomechanical consequences of varus valgus alignment are known, but the relationship between the altered mechanics and PF osteoarthritic disease has been minimally examined. The current results are consistent with the concept that varus valgus alignment affects the stresses sustained by the PF joint. However, although a greater proportion of those with lateral PF OA had valgus knees, the proportion with varus knees was larger than might have been expected. Harrison et al have described lateral PF changes in subjects with varus knees and have theorized that, in some patients, varus malalignment may lead to relative femoral internal rotation caused by loss of medial joint space, leading to impingement of the lateral femoral condyle on the lateral patellar facet (21). We found a corresponding greater frequency of varus malalignment in those with medial PF OA, regardless of which definition of medial PF OA was used. This is consistent with the biomechanical consequences of varus malalignment for the Q-angle. A small subset of

6 VARUS-VALGUS ALIGNMENT AND PF OA 1879 patients with medial PF OA had valgus malalignment. Harrison et al did not find a medially positioned patella in any valgus knee (21). Our results suggest that varus valgus alignment has a relationship with compartment involvement in PF OA. It is likely that rotational malalignment or torsion is also important. Torsion alters PF pressure distribution and may lead to articular cartilage damage (22). An association between increased femoral anteversion and PF OA has been reported (23). Static measures of varus valgus alignment or torsion do not account for the contributions to PF mechanics made by joint instability, muscle activity, or individual idiosyncrasies of activity performance. There is a paucity of epidemiologic information regarding the natural history of PF OA and the factors linked to its development and progression. Populationbased studies often have not included PF compartment visualization or have relied on the lateral radiograph of the knee. Because the lateral view does not allow determination of which PF compartment is involved, there are no population-based data with which we can compare our results. Available information for PF OA originates from cross-sectional studies. Factors linked to the presence of PF OA include age (5), sex (5), knee injury (5,24), Heberden s nodes (24), BMI (5,25), elite weight lifting (26), and elite running or tennis (27). In a clinic-based study, 28% of those with isolated PF OA, but none with combined PF-TF OA, had a history of patellar dislocation or subluxation (6). There was no evidence of joint hypermobility syndrome in patients with PF OA (6). Factors that influence progression of established PF OA have not been identified. Limitations of the current study include the fact that it is not population based. However, subjects were recruited from several sources in the community to maximize the generalizability of the results. An osteoarthritic cohort may be especially useful for examining questions that, because of measurement time or expense, are less feasible in population-based studies (e.g., semiflexed, fluoroscopy-confirmed knee radiographs, full-limb views to assess alignment, or axial, weightbearing PF views). This study is cross-sectional; the role of malalignment in the development of PF OA should be further examined in a cohort free of PF OA at baseline. In conclusion, lateral PF OA was more common than medial PF OA. Valgus malalignment was associated with lateral PF OA, and varus malalignment was associated with medial PF OA. Knees with PF OA were more often valgus than knees with isolated TF OA. Varus or valgus malalignment may influence the risk of developing medial PF OA or lateral PF OA, respectively. REFERENCES 1. 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