Relation of the chondromalatia patellae to proximal tibial anatomical parameters, assessed with MRI

Chondromalacia patellae is a common reason for patellofemoral pain syndrome. It is defined as the disruption of patellar cartilage due to repeated stress to the articular surface.1 These patients may experience frequent recurrence and chronic pain, which limits daily life activities.2 Radiographs were traditionally used to diagnose chondromalacia; however, this modality was unable to visualize patellar alignment and congruency angles3, and the complete anatomy was not visualized using X-ray.

Magnetic resonance imaging (MRI), a non-invasive tool to detect chondromalacia patellae with a high soft tissue contrast, can detect chondromalacia in the lower stages with a sensitivity of 66%. Moreover, this sensitivity rises to 85–100% for the higher stages.4 Many key findings can be detected in the early stage of chondral loss including signal irregularities, fissures, and chondral thinning; thus, leading to earlier diagnosis.5

Multiple mechanisms can cause chondromalacia, including vascular insufficiency, trauma, and structural abnormalities.6 In line with the mechanism of structural abnormalities, many studies have evaluated patellar and femoral trochlear morphology with different radiologic indices. Femoral intertrochlear notch angle, femoral trochlear depth, femoral sulcus angle, femoral trochlear angle, patellar tilt angle, and patellar height indices were among these factors. Many patellar height indices, measured as patellar tendon to patellar length ratio, seemed to correlate with cartilage defects.7 Furthermore, the correlation between tibial structural abnormalities and patellar cartilage stress, but not cartilage defects, was evaluated previously.8 On the other hand, some studies focused on the correlation between soft tissue structure and chondromalacia, particularly subcutaneous fat and adjacent muscles.8, 9

Radiologic findings of the knee joint differ between low-grade (grade 1, 2) and high-grade (grade 3,4) chondromalacia. For instance, the sulcus angle and trochlear depth were shown to be significantly different between mild and severe cartilage defects.10

The role of tibial anatomy is not well-known in developing chondromalacia. Some studies evaluated the role of tibial anatomy in dynamic models, but not in human subjects.11

To our knowledge, there is no published diagnostic model of radiological findings for chondromalacia that includes all important findings including tibial anatomical parameters. We aimed to develop a model and assess the discriminative power of tibial, patellar, and femoral indices for chondromalacia and emphasize the role of tibial anatomy for the first time. We also hypothesize that patellar height is a substantial factor, affecting the development of the disease.

Prediction models are becoming more popular in medicine. Previous studies focused their attention on the correlation between trochlear morphology and chondromalacia patellae, in a single variable analysis model. We developed a model to predict chondromalacia. We included medial tibial slope, lateral trochlear inclination, lateral patellar tilt angle, trochlear depth, and age in the final predictive model for chondromalacia. We found that any increase in lateral trochlear inclination and lateral patellar tilt angle could increase the probability of the disease, while any increase in medial tibial slope and trochlear depth could decrease the probability of the disease. We also found that if patellar height increases, patients would experience more severe disease (higher grades).

Lateral patellar tilt angle (LPTA) was suggested as one of the predictive factors in the final model. LPTA was shown to be increased in chondromalacia subjects. Abnormal position of the patella in the femoral trochlear groove may play a role in the progression of chondromalacia patellae.21, 22 Studies have shown that lateral patellar displacement is associated with cartilage loss.23 This can be justified by the fact that lateral tilting happens as a result of lateral compressive forces which acts on the lateral facet and increases shear forces in the central ridge area. These forces will increase chondrocyte activity, until the functional requests exceed chondrocyte potential and results in chondrocyte degeneration.24 Moreover, previous studies have found that LPTA correlates with the sulcus angle in different knee flexion angles.25 Sulcus angle has been reported to be greater in lateral maltrackers compared to non-lateral maltrackers26, and it also correlates with cartilage lesions.27 A wider sulcus is responsible for the increased pressure on patellofemoral articular surface and will predispose the patient to cartilage loss.27 This correlation between the sulcus angle and LPTA may justify our result. However, LPTA has been reported to correlate significantly with chondral lesions in the lateral compartment of the patellofemoral joint (odds ratio < 1) and non-significantly with chondral lesions in the medial compartment of the patellofemoral joint (odds ratio > 1) in previous studies.27 In this study, we did not define the side of cartilage defect. As chondromalacia impacts the medial side more frequently28, we may speculate that this relationship in our study should be judged according to the side of involvement.

In addition, LTI was positively correlated with chondromalacia according to our results. Kuroda et al. showed that elevation of lateral trochlea facet (increased LTI) could increase average patellofemoral contact pressure (40% increased with 10 mm lateral trochlear elevation). This may also induce the degradation of the cartilage.29

The most important finding of the present study is that the tibial slope may play an important role in the progression of chondromalacia. We found that with the medial tibial slope increasing, the risk of chondromalacia decreases. Studies suggest that an increase in the tibial slope leads to a decrease in patellofemoral contact force.30 Posterior positioning of femoral components and increase in quadriceps lever arm are responsible for this association.31, 32, 33 We deduce that the tibial slope correlates with chondromalacia patellae; however, the effect of tibial slope on medial and lateral patellar facets may be different. Furthermore, correcting the tibial slope with a greater angle has been shown to reduce anterior tibial strains, which are imposed on the patella.34 Moreover, studies in patients with osteoarthritis suggested that increasing the tibial slope in unicompartmental knee arthroplasty of medial patellar facet will reduce the tension on the medial side.11 This may also show the clinical importance of tibial slope on decreasing patellofemoral stress.

Our study suggests that trochlear depth would be sufficient for measuring the association of trochlear dysplasia with chondromalacia. Sulcus angle and trochlear depth are the two important factors associated with trochlear dysplasia in previous studies35, and the congruency of the patella and femoral trochlea has been underscored in the development of cartilage lesions.10 Patellofemoral tracking is also considered an important factor in patellofemoral pain syndrome.36 Trochlear depth has been reported to be an important determinant of patellofemoral stability37, thus, it may be an important factor in the process of tracking. Tuna et al. reported sulcus angle and trochlear depth to be significantly different in patients with and without the disease.18 They reported greater mean of sulcus angle and lower mean of trochlear depth in chondromalacia subjects. Our findings are consistent with the latter study. We found that with the trochlear depth decreasing, the risk of chondromalacia increases to a great extent. This may be explained by the fact that when the trochlear surface becomes shallower, the surface area for articulation at the patellofemoral joint increases. Although it may provide a better distribution of joint load37, the increased friction may result in cartilage defects. Further studies are needed to evaluate this proposed kinematics in active knee movements. This finding may underscore the clinical importance of trochlear depth. Takahashi et al. also showed better adaptation of the implants featuring deep trochlea to the native patella (low-contact stress implant and NexGen implant).38

We also proposed a predictive model of disease severity. In our study, ISI and age could predict higher disease severity. To our knowledge, this is the first predictive model for the severity of chondromalacia with an excellent AUC. A model was proposed in the previous study by Lu et al. for chondromalacia but not severity by using Insall-Salvati index (ISI). This model was based on the patella alta and baja and could diagnose chondromalacia with an AUC of 0.596.7

In both predictive models proposed by our findings, age was remained as an important factor. A study in the United States population showed that the contribution of chondromalacia to the patellofemoral pain syndrome increases by age up to 59 years.39 We also excluded patients older than 65 years and the results may not be applicable to older age groups.

There are some limitations to our study. This was a single center study, we used MRI to detect chondral lesion and early lesions might be considered as normal. MRI evaluations were performed during rest. Muscle contractions may alter these findings as described by previous studies.42 We did not evaluate the exact site of the cartilage lesions. Moreover, medial and lateral facet chondromalacia might have different natures. Previous studies reported the predominance of chondromalacia in the medial patellar facet.22 We did not define the side of chondromalacia in our subjects, so more pieces of evidence are needed to define the exact correlation of tibial slope with both medial- and lateralfacet chondromalacia.

The advantage of this study in knowledge is that when more diverse anatomical factors are analyzed regarding the presence of chondromalacia, tibial anatomical factors play a role in the progression of the disease. This study may clarify the mechanism of chondromalacia and emphasize the role of tibial slopes in the mechanism of cartilage loss. Previous studies focused on femoral trochlear dysplasia, and did not include tibial anatomical parameters in their analysis. The result of this study meight also affect the treatment of chondromalacia, as new treatment plans for healing cartilage and bone defects are being introduced and focus on altering tibial anatomy including tubercle osteotomy.40, 41 The result of this study shew the role of tibial anatomy in developing chondromalacia. In addition, the clinical importance of this study was the fact that tibial slope should be precisely set in knee surgeries; however, more studies are needed to determine the best cut-off for tibial slope.