Abstract
Objective
To examine the relationship between patella alta and the prevalence and worsening at follow-up of structural features of patellofemoral joint (PFJ) osteoarthritis (OA) on MRI.
Methods
The Multicenter Osteoarthritis (MOST) Study is a cohort study of persons aged 50-79 years with or at risk for knee OA. Patella alta was measured using the Insall-Salvati ratio (ISR) on the baseline lateral radiograph and cartilage damage, bone marrow lesions (BMLs), and subchondral bone attrition (SBA) were graded on MRI at baseline and at 30 months follow-up in the PFJ. We examined the association of the ISR with the prevalence and worsening of cartilage damage, BMLs, and SBA in the PFJ using logistic regression.
Results
907 knees were studied (mean age 62, BMI 30, ISR 1.10), 63% from female subjects. Compared with knees in the lowest ISR quartile at baseline, those in the highest had 2.4 (95% CI 1.7, 3.3), 2.9 (2.0, 4.3), and 3.5 (2.3, 5.5) times the odds of having lateral PFJ cartilage damage, BMLs, and SBA respectively, and 1.5 (95% CI 1.1, 2.0), 1.3 (0.9, 1.8), and 2.2 (1.4, 3.4) times the odds of having medial PFJ cartilage damage, BMLs, and SBA respectively. Similarly, those with high ISRs were also at risk for worsening of cartilage damage and BMLs over time than those with low ISRs.
Conclusion
A high ISR, indicative of patella alta, is associated with structural features of OA in the PFJ. Additionally, the same knees have increased risk of worsening of these same features over time.
Symptomatic knee osteoarthritis (OA) is present in 12% of individuals over the age of 60 and is a leading cause of disability in the United States (1, 2). Research has focused on risk factors for tibiofemoral joint OA, but both symptoms and disease often occur in the patellofemoral joint (PFJ) (3-6). Despite growing awareness of PFJ OA as a common source of pain and functional limitation among older adults, there continues to be paucity of literature on its risk factors. Those studies that have investigated risk factors have focused on patellar tilt and displacement, femoral trochlea morphology (7-9), and frontal plane tibiofemoral alignment (10, 11).
Patella alta, or a “high riding” patella, is a distinct feature of PFJ alignment that has been proposed to be a risk factor for patellofemoral pain and OA (12-14). Patella alta is measured by the Insall-Salvati ratio (ISR), a ratio between the length of the patellar tendon and the length of the patella (15), with a ratio ≥1.2 indicating patella alta. Ward et al. (14, 16, 17) found subjects with patella alta to have increased PFJ malalignment, decreased PFJ contact area, and increased PFJ stress. These findings suggest that persons with patella alta may be at increased risk for damage of the PFJ cartilage and underlying bone. Similarly, in studies using dynamic knee simulators and cadaveric knees, PFJ contact forces were higher in knees with patella alta compared to knees with normal patellar position (13, 18).
Magnetic resonance imaging (MRI) directly visualizes structural features of OA including focal cartilage damage, bone marrow lesions (BMLs), and subchondral bone attrition (SBA). These features can be graded specifically for the medial and lateral patella and trochlea facets yielding a comprehensive picture of compartment specific PFJ structural change. While BMLs and SBA have been shown to be associated with pain, occur in the same subregions as cartilage damage, and alignment factors affecting loading at the tibiofemoral joint (19-24), few studies have investigated their relationship with alignment of the PFJ.
The small number of cross-sectional studies that have explored the relationship of patella alta to MRI features of PFJ OA have yielded conflicting results. Ali et al. found no differences in the ISR among groups with no, mild, or severe patellofemoral articular cartilage damage (25). In contrast, Kalichman (8) used a more detailed method of defining cartilage damage and BMLs and found an association between the modified ISR (ModISR) (26) and cartilage damage in the lateral and medial PFJ and BMLs in the lateral PFJ.
To our knowledge there have been no longitudinal studies investigating this question. The purpose of this study was to determine the association between patella alta and the prevalence and worsening of PFJ cartilage damage, BMLs, and SBA in middle aged and older persons with or at risk for knee OA. We hypothesized that knees with patella alta would have a greater baseline prevalence and greater worsening at follow-up of cartilage damage, BMLs, and SBA in the PFJ.
Materials and Methods
Study Population
The Multicenter Osteoarthritis (MOST) Study is a prospective cohort study of 3,026 individuals, aged 50 to 79 years, who either have or are at high risk of knee OA. Subjects were recruited from 2 communities in the United States: Birmingham, Alabama and Iowa City, Iowa. Details of the study population have been published elsewhere (22). In the present study, we used a sample of 907 knees, one knee per subject, randomly selected from knees which underwent MRI at baseline and 30 months and whose MRIs were read for structural abnormalities as part of other completed studies. In all subjects, lateral radiographs were acquired at baseline. The institutional review boards at the University of Iowa, University of Alabama, Birmingham, University of California, San Francisco, and Boston University School of Medicine approved the study protocol.
Patella alta assessment
The ISR (15) and ModISR (26) were measured from the baseline lateral semi-flexed weight bearing radiograph (Figure 1). We used the weight bearing lateral radiograph as opposed to sagittal MRI because the patellar position can change with knee flexion. This procedure was standardized across all patients and study sites (27) and yielded knee flexion angles of 20-50 degrees (median 39 degrees). All measurements were made using Osirix digital software (Version 3.2.2). The ISR was calculated by dividing the distance from the tibial tuberosity to the inferior pole of the patella by the length of the patella from the apex of the patella to the most posterior superior point. The ModISR was calculated by dividing the distance from the tibial tuberosity to the inferior margin of the articular surface of the patella by the length of the articular surface of the patella. We additionally calculated a ratio between the length of the patellar tendon and the subjects' height (LTH).
Figure 1.
The Insall-Salvati Ratio was calculated by dividing the length of the patellar tendon (LT) by the length of the patella (LP) measured from the lateral radiograph (15). The modified Insall-Salvati Ratio was calculated by dividing the distance from the tibial tuberosity to the inferior pole of the articular surface of the patella (LT2) by the length of the articular surface of the patella (LA) (26).
PFJ alignment assessment
Two additional measures of PFJ alignment, patellar tilt and lateral displacement, were measured on axial MRI images (28, 29). Bisect offset (BO) is a measure of patellar lateral displacement (% of the patella lateral to the midline of the trochlea). Patellar tilt angle is the angle between a line connecting the posterior femoral condyles and a line defining the maximal patellar width. First, a posterior condylar line (PCL) is drawn along posterior femoral condyles on the axial MRI slice where femoral condyles are most posterior, then a line is drawn through the center of the trochlea perpendicular to the PCL (Figure 2, panel A). On the slice where patellar width is maximal, a line is drawn connecting medial and lateral margins, the PCL is copied to this slice, and the patellar tilt angle is the angle between the PCL and the maximal patellar width line (Figure 2, panel B). The perpendicular line through the center of the trochlea is copied to the maximum patellar width slice, and BO = (length of patellar lateral to midline/maximal patellar width)*100 (Figure 2, panel C).
Figure 2.
Patellofemoral alignment (patellar tilt angle and bisect offset {BO}) was measured using images from the axial MRI. First, a posterior condylar line (PCL) is drawn along posterior femoral condyles on the axial MRI slice where femoral condyles are most posterior, and then a line is drawn through the center of the trochlea perpendicular to the PCL (panel A). On the slice where patellar width is maximal, a line is drawn connecting medial and lateral margins, the PCL is copied to this slice, and the patellar tilt angle is the angle between PCL and maximal patellar width line (panel B). The perpendicular line through the center of the trochlea is copied to the maximum patellar width slice and BO = (length of patellar lateral to midline/maximal patellar width)*100 (panel C).
Outcome assessment
A 1.0 Tesla OrthOne™ scanner was used with a phased array knee coil to acquire axial and sagittal MRIs (proton density fast spin echo fat suppressed) and coronal short tau inversion recovery (STIR) sequences of all eligible knees at baseline and 30-month visits. Two musculoskeletal radiologists (FR, AG) used the Whole-Organ Magnetic Resonance Imaging Score (WORMS) to assess cartilage damage at the medial and lateral patellar and trochlear facets (30). The cartilage scale ranges from 0-6: 0=normal cartilage morphology; 1=normal thickness but increased signal on T2-weighted images; 2.0=partial thickness focal defect <1 cm in greatest width; 2.5=full thickness focal defect <1 cm in greatest width; 3=multiple areas of partial-thickness (Grade 2.0) defects intermixed with areas of normal thickness, or a Grade 2.0 defect wider than 1 cm but <75% of the region; 4=diffuse (≥75% of the region) partial-thickness loss; 5=multiple areas of full thickness loss (grade 2.5) or a grade 2.5 lesion wider than 1 cm but <75% of the region; 6=diffuse (≥75% of the region) full-thickness loss. BMLs and SBA were also assessed using the WORMS method, in the subchondral bone of the patella and trochlea in the medial and lateral compartments of the PFJ. The BML scores range from 0-3: 0=normal, 1=small, <25% of region, 2=medium, 25% - 50% of region, 3=large, >50% of region. SBA was defined as flattening or depression of the articular surfaces graded from 0 to 3 based on the degree of deviation from the normal contour: 0=normal; 1=mild; 2=moderate; 3=severe. For longitudinal WORMS readings, a score for a within grade change (+0.5) was used for cartilage and (±0.5) BMLs to account for changes that did not fulfill the definition of a full-grade change.
Reliability assessment
All measurements of patella alta and PFJ alignment were repeated by the primary reader (JS) and a second reader (AZ) in 10% of all knees to determine inter and intra-rater reliability. Inter and intra-rater ICCs for patellar tendon and patellar lengths, ISR, BO, and patellar tilt angle were ≥0.9; inter and intra-rater ICCs for articular surface length were 0.8 and 0.92 and for ModISR were 0.7 and 0.93, respectively. Inter-reader weighted kappa values for WORMS scores ranged from 0.62-0.78.
Statistical analysis
We first categorized the ISR, ModISR, and LTH into quartiles to determine the relationship between multiple levels of the predictors and outcomes. Both patellar and trochlear subregions were included in analyses for the medial and lateral PFJ outcomes; for 907 knees there are 1814 subregions used for the medial and lateral PFJ. For the prevalence analysis, we dichotomized cartilage damage (≥2), BMLs (≥1), and SBA (≥1) into presence or absence of pathology. Cartilage grades 0 and 1 were considered no damage because grade 1 represents a change in signal but no change in thickness. For the longitudinal analysis, subregions with any increase WORMS score for cartilage morphology, BMLs, and SBA were considered to have worsened. BMLs with no change or improvement in scores were considered not to have worsened. Subregions with a maximum WORMS score at baseline were excluded from this analysis. To determine the relationship between the predictors and cartilage damage, BMLs, and SBA in the PFJ at baseline and over 30 months, we used logistic regression with generalized estimating equations to account for the correlation between WORMS scores for patellar and trochlear subregions from the same knee. All analyses were adjusted for age, sex, and body mass index (BMI). We also tested for linear trend by using the predictors as continuous variables in all models. Statistical analyses were performed using SAS software (SAS Institute Inc, Cary, NC, USA, version 9.1). In separate analyses we included BO and patellar tilt angle in the models to determine the independent association between the predictors and outcomes.
Results
Of the 907 knees selected for study at baseline, 9 had missing ISRs, 10 had missing SBA readings at baseline or 30 months, 2 had missing cartilage scores at baseline, and 2 had missing BMLs scores at baseline or 30 months. At baseline the mean age, BMI, ISR, ModISR, bisect offset, and patellar tilt angle was 62, 30, 1.10, 1.74, 60.1, and 10.2, respectively; 20% had PFJ OA on x-ray (31) and 63% of the knees studied were from female subjects (Table 1). The range of the highest quartile of the ISR was 1.22-1.66, which corresponds closely with previous definition of patella alta being ISR ≥ 1.2 (15). Of the subregions studied at baseline, 38% had cartilage damage, 22% BMLs, and 17% SBA in the lateral PFJ and 49% had cartilage damage, 19% BMLs, and 14% SBA in the medial PFJ (Table 2). Subregions of knees with maximum possible WORMS scores at baseline could not worsen and were excluded from analysis of longitudinal outcomes (143/1812 (8%) of subregions for cartilage scores, 51/1813 (3%) for BMLs, and 39/1796 (2%) for SBA in the lateral PFJ and 49/1812 (3%) for cartilage scores, 14/1813 (<1%) for BMLs, and 6/1796 (<1%) for SBA in the medial PFJ).
Table 1. Participant Characteristics (n=907 knees).
| Age mean ± SD years | 62 (8.0) |
| Female (%) | 63% |
| PFJ OA (%) | 20% |
| BMI mean ± SD kg/m2 | 30 (4.8) |
| ISR* mean ± SD | 1.1 (0.17) |
| ISR range | 0.55-1.66 |
| BO mean ± SD | 59.9 (10.2) |
| Patellar tilt angle mean ± SD degrees | 10.2 (6.5) |
ISR missing from 9 knees
Table 2. Frequency of structural outcomes at baseline and worsening at 30 months.
| Baseline frequency | Worsening frequency | |
|---|---|---|
| Lateral PFJ* | ||
| Cartilage damage | 680/1812 (38%) | 143/1669 (9%) |
| BMLs | 403/1813 (22%) | 143/1760 (8%) |
| SBA | 312/1796 (17%) | 17/1755 (<1%) |
| Medial PFJ* | ||
| Cartilage damage | 881/1812 (49%) | 142/1763 (8%) |
| BMLs | 353/1813 (19%) | 89/1797 (5%) |
| SBA | 259/1796 (14%) | 13/1788 (<1%) |
Outcomes combine the scores from the patella and trochlea, as described in methods.
In the lateral PFJ, compared with knees in the lowest ISR quartile, those in the highest quartile had 2.4 to 3.5 fold increased risk of cartilage damage, BMLs, and SBA (Table 3). The likelihood of cartilage damage, BMLs, and SBA increased with higher ISRs (p for trend <0.0001). When adjusting for BO and patellar tilt angle, compared with knees in the lowest ISR quartile, those in the highest quartile had 1.5 (1.1, 2.2), 1.9 (1.3, 2.9), and 2.0 (1.2, 3.2) times the odds of having lateral PFJ cartilage damage, BMLs, and SBA, respectively. A test for linear trend remained significant for cartilage damage (p=0.0016), BMLs (p=0.0001), and SBA (p=0.0009).
Table 3. The association between quartiles of the Insall-Salvati Ratio and baseline PFJ cartilage damage, BMLs, and SBA.
| Quartile 1 | Quartile 2 | Quartile 3 | Quartile 4 | p for trend | |
|---|---|---|---|---|---|
| ISR Range | 0.55-0.98 | 0.99-1.09 | 1.10-1.21 | 1.22-1.66 | |
| # of knees | 220 | 222 | 232 | 224 | |
| Lateral PFJ Cartilage Damage | |||||
| WORMS ≥2 x/n (%) | 129/439 (29) |
147/443 (33) |
169/464 (36) |
223/448 (50) |
|
| Adjusted OR* (95% CI) |
1.0 (reference) |
1.2 (0.8, 1.7) |
1.3 (1.0, 1.9) |
2.4 (1.7, 3.3) |
<0.0001 |
| Bone Marrow Lesions | |||||
| WORMS ≥1 x/n (%) | 67/440 (15) |
78/443 (18) |
94/464 (20) |
153/448 (34) |
|
| Adjusted OR* (95% CI) |
1.0 (reference) |
1.2 (0.8, 1.8) |
1.4 (0.9, 2.1) |
2.9 (2.0, 4.3) |
<0.0001 |
| Subchondral Bone Attrition | |||||
| WORMS ≥1 x/n (%) | 45/434 (10) |
61/440 (14) |
66/462 (14) |
131/442 (30) |
|
| Adjusted OR* (95% CI) |
1.0 (reference) |
1.3 (0.8, 2.2) |
1.4 (0.8, 2.2) |
3.5 (2.3, 5.5) |
<0.0001 |
| Medial PFJ Cartilage Damage | |||||
| WORMS ≥2 x/n (%) | 195/439 (44) |
201/443 (45) |
233/464 (50) |
243/448 (54) |
|
| Adjusted OR* (95% CI) |
1.0 (reference) |
1.0 (0.7, 1.4) |
1.2 (0.9, 1.6) |
1.5 (1.1, 2.0) |
0.0085 |
| Bone Marrow Lesions | |||||
| WORMS ≥1 x/n (%) | 74/440 (17) |
81/443 (18) |
106/464 (23) |
90/448 (20) |
|
| Adjusted OR* (95% CI) |
1.0 (reference) |
1.1 (0.8, 1.6) |
1.5 (1.1, 2.1) |
1.3 (0.9, 1.8) |
0.04 |
| Subchondral Bone Attrition | |||||
| WORMS ≥1 x/n (%) | 42/434 (10) |
64/440 (15) |
63/462 (14) |
86/442 (19) |
|
| Adjusted OR* (95% CI) |
1.0 (reference) |
1.5 (1.0, 2.4) |
1.4 (0.9, 2.2) |
2.2 (1.4, 3.4) |
0.0001 |
Adjusted for age, sex, and BMI
In the medial PFJ, compared with knees in the lowest ISR quartile, those in the highest quartile had a 1.3-2.2 fold increased risk of cartilage damage, BMLs, and SBA (Table 3). A test for trend also revealed a linear relationship, though not as significant as the association between increasing ISR and lateral PFJ, for cartilage damage (p=0.0097), BMLs (p=0.04), and SBA (p=0.0002). When adjusting for BO and patellar tilt angle, compared with knees in the lowest ISR quartile, those in the highest quartile had 1.3 (0.95, 1.8), 1.5 (1.0, 2.1), and 2.0 (1.3, 3.2) times the odds of having medial PFJ cartilage damage, BMLs, and SBA, respectively. A test for linear trend remained significant for BMLs (p=0.005) and SBA (p=0.0027) but not for cartilage damage (p=0.14).
When followed over time, worsening of lateral PFJ cartilage damage scores occurred in 9% of subregions, BMLs in 8% of subregions, and SBA in <1% of subregions (Table 2). Compared to those knees in the lowest ISR quartile, those in the highest quartile had 2.1 (1.2, 3.5) and 2.3 (1.2, 4.3) times the odds of having worsening cartilage damage and BMLs, respectively (Table 4). When adjusting for BO and patellar tilt angle, compared with knees in the lowest ISR quartile, those in the highest had 1.7 (1.0, 2.9) and 1.7 (0.9, 3.2) times the odds of worsening lateral PFJ cartilage damage and BMLs, respectively. A test for linear trend remained significant for cartilage damage (p=0.01) but not for BMLs (p=0.07).
Table 4. The association between quartiles of the Insall-Salvati Ratio and worsening PFJ cartilage damage and BMLs at follow-up.
| Quartile 1 | Quartile 2 | Quartile 3 | Quartile 4 | p for trend | |
|---|---|---|---|---|---|
| ISR Range | 0.55-0.98 | 0.99-1.09 | 1.10-1.21 | 1.22-1.66 | |
| # of knees | 220 | 222 | 232 | 224 | |
| Lateral PFJ Cartilage Damage | |||||
| WORMS change ≥0.5 x/n (%) | 27/427 (6) |
24/420 (6) |
42/434 (10) |
49/379 (13) |
|
| Adjusted OR* (95% CI) |
1.0 (reference) |
0.9 (0.5, 1.6) |
1.5 (0.9, 2.5) |
2.1 (1.2, 3.5) |
0.0009 |
| Bone Marrow Lesions | |||||
| WORMS change ≥0.5 x/n (%) | 21/433 (5) |
34/434 (8) |
38/451 (8) |
47/427 (11) |
|
| Adjusted OR* (95% CI) |
1.0 (reference) |
1.6 (0.9, 3.0) |
1.7 (0.9, 3.2) |
2.3 (1.2, 4.3) |
0.0009 |
| Medial PFJ Cartilage Damage | |||||
| WORMS change ≥0.5 x/n (%) | 20/436 (5) |
36/424 (8) |
44/454 (10) |
40/432 (9) |
|
| Adjusted OR* (95% CI) |
1.0 (reference) |
1.9 (1.0, 3.4) |
2.1 (1.2, 3.7) |
2.0 (1.1, 3.6) |
0.005 |
| Bone Marrow Lesions | |||||
| WORMS change ≥0.5 x/n (%) | 14/436 (3) |
28/439 (6) |
26/461 (6) |
21/444 (5) |
|
| Adjusted OR* (95% CI) |
1.0 (reference) |
2.0 (1.0, 3.8) |
1.7 (0.9, 3.3) |
1.4 (0.7, 2.8) |
0.45 |
Adjusted for age, sex, and BMI
In the medial PFJ, worsening of cartilage damage occurred in 8% of subregions, BMLs in 5% of subregions, and SBA in <1% of subregions (Table 2). Compared to those knees in the lowest ISR quartile, those in the highest had 2.0 (1.1, 3.6) and 1.4 (0.7, 2.8) times the odds of worsening cartilage damage and BMLs, respectively (Table 4). Since worsening of SBA only occurred <1% of the time, we were unable to calculate stable effect estimates for worsening and do not report them here. When adjusting for BO and patellar tilt angle, compared with knees in the lowest ISR quartile, those in the highest had 2.1 (1.1, 3.8) and 1.4 (0.7, 2.8) times the odds of worsening of lateral PFJ cartilage damage and BMLs, respectively. A test for linear trend remained significant for cartilage damage (p=0.0024) but not for BMLs (p=0.48).
We performed additional analyses stratifying our results by those with and without PFJ OA on x-ray at baseline, and the results were similar for all outcomes. Of note, 20 of 896 (2%) of knees studies had an ISR < 0.80, or patella baja. Since these knees might not be categorized as ‘normal’, we carried out a sub-analysis, removing those knees, and our results regarding risk of patella alta did not change.
We also found no association between the ModISR and any of our outcomes. However the new measurement, length of patellar tendon divided by height (LTH), showed significant associations with our outcomes. Specifically, for the cross-sectional analysis, compared to those in the lowest LTH quartile, those in the highest quartile had 2.3 (1.6, 3.2), 3.2 (2.2, 4.6), and 2.8 (1.8, 4.3) times the odds of having lateral PFJ cartilage damage, BMLs, and SBA, respectively, and 1.3 (0.9, 1.7), 1.4 (1.0, 2.1), and 1.7 (1.1, 2.7) times the odds of having medial PFJ cartilage damage, BMLs, and SBA, respectively. Similar results were seen for the association between LTH and worsening outcomes as were seen with the ISR.
Discussion
Our findings indicated that patella alta, measured by the ISR, was associated with an elevated prevalence of OA structural features in the medial and lateral PFJ. Additionally, compared to knees with low ISRs, knees with high ISRs had roughly twice the odds of worsening cartilage damage in the medial and lateral PFJ and BMLs in the lateral PFJ over 30 months. When adjusting for measures of PFJ alignment, there remained an independent association between the ISR and prevalence of structural damage on MRI.
The prevalence of PFJ OA in our 907 knees (20%) was similar to the entire MOST (23%) and Framingham (24%) cohorts. Of those with PFJ OA at baseline (20% of sample), 39% had patella alta (using ISR ≥1.2 as definition of patella alta) and of those without PFJ OA at baseline, 26% had patella alta. The excess of those with patella alta (13%) may be the proportion of PFJ OA attributable to patella alta.
The effect of the ISR on structural damage was greater in the lateral compartment compared to the medial. This may be due to the lateral directed force on the patella when the quadriceps contracts. Biomechanically, this laterally directed force on the patella (secondary to the Q-angle) results in increased force through the lateral PFJ. This increased force would be expected especially in knees with patella alta because they have less osseous stability, allowing more lateral displacement of the patellar during quads contraction. The strength of the association for the middle two quartiles of the ISR on our outcomes was weaker than the association between the highest ISR quartile. The highest ISR quartile in our study began at 1.22, suggesting that not only is 1.2 an appropriate definition for patella alta (15), but also is associated with structural damage on MRI.
Our findings support the hypothesis that knees with patella alta experience increased PFJ stress due to decreased contact area (14, 16). Ward et al. found that compared to subjects without patella alta, those with patella alta had decreased PFJ contact area at multiple degrees of flexion. Also, since subjects with patella alta have similar joint reaction forces, this decreased contact area results in greater PFJ stress during fast walking compared to subjects without patella alta. Similarly one study using a dynamic knee simulator (13) and another using cadaveric knees (18) found that PFJ contact forces were highest in knees with patella alta compared to normal patella position. In addition to increased compressive forces, subjects with patella alta may experience increased shear forces. Patella alta has been shown to be associated with lateral patellar subluxation (32, 33) and resultant shear forces may contribute to structural damage seen in these subjects.
Kalichman et al. measured patella alta using the ModISR on sagittal MRI images with the quadriceps relaxed and knee extended (8). Compared to knees in the lowest ModISR quartile, knees in the highest quartile had 2.0 and 2.5 times the odds of having lateral cartilage damage and BMLs and 2.0 and 1.1 times the odds of having medial cartilage damage and BMLs. Additionally, they found a U-shaped relationship between the ModISR and lateral BMLs (p=0.007). Conversely, our data demonstrated a strong linear relationship (p<0.0001) between quartiles of the conventional ISR and outcomes in the lateral PFJ and a moderate/weak linear relationship in the medial PFJ (Table 3). We also measured the ModISR on the lateral radiograph (26) and found no association with any outcome in either the medial or lateral PFJ.
Contrary to our findings that patella alta was strongly associated with cartilage damage in the medial and lateral PFJ measured on MRI, several studies have shown no association between patella alta and cartilage damage on MRI (25) and arthroscopy (34-37). These differences in results may be attributed to different methods of cartilage assessment and to differences in study populations. Ali et al. (25) did not clearly define cartilage damage on MRI, nor damage in specific anatomic regions of the PFJ (i.e. medial vs. lateral, patella vs. trochlea). They found no differences in the ISR among groups with none, mild, and severe damage, but did find the ModISR to be greater (p=0.02) in the group with no damage (1.87) compared to the severe group (1.71). These results suggest that individuals with a higher riding patella ironically have less damage than those without it despite biomechanical evidence suggesting a higher risk for those with patella alta (13, 14, 18). In our study the mean age was 62 whereas in previous studies mean ages were no greater than 45 years. Also, we measured cartilage damage on both the medial and lateral patella and trochlea, while these studies made no attempt to localize damage to any specific anatomical region. Additionally the statistical analyses in these studies did not include potential confounding variables that are common in OA studies (i.e. age, sex, and BMI) while others used t-tests to compare multiple groups (none, mild, severe cartilage damage).
Patella alta is a potential cause of PFJ malalignment due to the lack of the osseous stability provided by the femoral trochlea especially when the knee is extended. Thus, PFJ malalignment is may be part of the causal pathway between patella alta and structural damage in the PFJ. Even so, our results suggest that the ISR is associated with the presence of structural damage in the PFJ regardless of the alignment of the patella. After adjusting for two measures of PFJ alignment, a significant association remained between the ISR and all of our outcomes, except baseline medial cartilage damage and worsening of medial and lateral BMLs. Ward et al. (14) found that compared to normal subjects, those with patella alta had more lateral tilt and displacement on MRI at 0° flexion with the quadriceps contracted. Additionally, PFJ alignment was not associated with PFJ contact area while the ISR was, leading the authors to challenge the long accepted cause effect relationship between PFJ alignment and PFJ pathology. Despite there being an association between the ISR and PFJ alignment, we suspect that patella alta may cause PFJ malalignment, and since it is potentially part of the causal pathway the results presented in tables 3 and 4 are not adjusted for PFJ alignment.
There are some limitations to the current study. First, we recognize that bone attrition is difficult to assess on the patella, especially since the lateral patella is normally concave. If we change our definition of SBA to a WORMS score ≥2, a score reflecting more severe bone attrition, our results still show a strong association between ISR and SBA in the lateral [OR 4.4 (2.4, 8.3)] and medial [OR 7.1 (1.7, 30.4)] PFJ. Second the measurement of the ISR is highly dependent on patellar morphology. To address this we used the ModISR but found no association with any outcomes. One reason for this null finding may be that the inter-rater ICC (0.7) was poorer compared to the ISR (.97), attributable possibly to excluding osteophytes from the measurement. Additionally, a high ISR could also be a result of a small patella. To address this we calculated a ratio between the length of the patellar tendon and the subjects' height and found similar results to those using the conventional ISR.
In summary, high ISRs were associated with cartilage damage, BMLs, and SBA in the medial and lateral PFJ. Additionally, these knees were at risk of worsening cartilage damage at follow-up in both the medial and lateral PFJ and BMLs in the lateral PFJ. Effect estimates were greater for the lateral PFJ, which is what we expected due to the forces acting on the lateral PFJ. The ISR is easily measured on the lateral semi-flexed weight-bearing radiograph, which can be obtained in the clinic. Although patella alta is not modifiable, physicians can identify those with high ISRs as high risk for PFJ OA and stress prevention. Future research is needed to determine the association between patella alta, pain, and function.
Acknowledgments
The MOST study is supported by the NIH (grants U01-AG18820, U01-AG18832, U01-AG18947, U01-AG19069, and AR-47785). Mr. Stefanik's work was supported by a doctoral dissertation award from the Arthritis Foundation.
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