The findings of this study confirmed our hypothesis that the width of medial and lateral trochlea was altered and the trochlear center was medialized in trochlear dysplasia measured by linear measurements. Compared with normal trochlear, the width of medial trochlea was significantly reduced in dysplastic trochlear, and the width of lateral trochlear was significantly increased. The distance of the medial edge of the trochlear facet relative to the medial epicondyle was increased in the TD group which indicated the narrowness of the medial trochlear facet. There was no significant difference in the distance of the lateral edge of trochlear facet between groups. Additionally, in the TD group, the medialization of the proximal trochlear groove was correlated with the increase in the lateral trochlear width.
The medial hypoplasia of trochlear facets featured with a flattened medial slope was recognized as one of the most distinctive features of trochlear dysplasia. Researchers had quantified the morphology of distal femur condyle and found that the decreased medial condylar height and width related to the hypoplastic medial trochlear were the major deformities of trochlear dysplasia . The reduced medial trochlear found by our study was consistent with the previous findings. In addition, the increased distance between the medial epicondyle and medial trochlear edge proposed that the hypoplasia of medial trochlear may be started from the medial edge of the trochlear facet.
In individuals, geometry of the dysplastic trochlear was complicated to be evaluated quantitatively . The anterior condylar height and trochlear groove depth were commonly used measurements to quantify the morphology of trochlear dysplasia . Ferlic et al. had reported excellent reliability of the linear measurements in quantifying the dysplastic trochlear . Some researchers had reported that a trochlear depth of less than 3 mm and a medial-to-lateral facet ratio of less than 40% were considered pathological in trochlear dysplasia . In this study, the tangent distance of trochlear edge and trochlear width was assessed relative to the anatomical trans-epicondylar axis. Compared with measuring the length of inclined trochlear slope, these measurements with the reference of medial epicondyle may directly reflect the relative orientation of the trochlear on the distal femoral condyle.
A tibial tubercle-trochlear groove distance of greater than 20 mm was commonly used as a surgical indicator for the medialization tibial tubercle osteotomy. Some people reported that increased tibial tuberosity-trochlear groove (TT-TG) distance was due to the medialization of trochlear groove by measuring the distance between the trochlear groove and the tibial landmarks . Some literature supported that the TT-TG distance was greater in recurrent patellar dislocation, but the extent of tibial tubercle lateralization was not substantial . The various reference points measuring the trochlear groove raised a problem that whether medialization of trochlear groove was caused by the trochlear dysplasia or by the torsion of lower limbs. Studies had reported that degrees of femoral torsion or knee torsion also influenced the measurement of tibial tuberosity-trochlear groove distance and patellofemoral congruence [3, 20, 21]. Tibial tubercle osteotomy was to transfer the tibia tubercle medially. The deepening trochleoplasty needed to remodel the trochlear groove and oriented the groove laterally. The primary deformities may have crucial influences on whether to perform tibial tubercle osteotomy or trochleoplasty accompanied by medial patellofemoral reconstruction. By using the reference points on the distal femoral condyle, the findings of our study demonstrated the medialization of trochlear groove in trochlear dysplasia besides the torsional factors.
Morphologically, trochlear dysplasia was featured by irregular surface from the proximal trochlear [3, 22]. In this study, the dTG was decreased relative to the medial epicondyle indicated that the medialization of the dysplastic trochlear groove. And the lateral trochlear width was increased in the dysplastic trochlear compared with the normal trochlear. Besides these morphological findings, the present study found a significant correlation between the lateral trochlear width and medialization of the trochlear center. Besides, there was no significant change in the position of lateral trochlear edge between groups, indicating that the position of lateral trochlear facets was not lateralized. Hence, it was speculated that the medial expansion of lateral trochlear was the reason for the medialization of the proximal trochlear groove in trochlear dysplasia.
The etiology of recurrent patellar instability was multifactorial [22,23,24]. Trochlear dysplasia was a major risk factor for patellar instability. The development of trochlear dysplasia was still controversial. Studies had reported that minor changes in trochlear morphology during skeletal growth and indicated that trochlear dysplasia was genetically determined [22, 25]. According to the researchers, the dysplastic trochlear could be surgically induced using the animal model of patellar subluxation [26, 27]. The trochlear bump and distance between the proximal trochlear and distal femoral physis increased with age which indicated the role of acquired factors on trochlear development . The skeletal growth could be strengthened by applied stress according to Wolff's law . The increased lateral trochlear in trochlear dysplasia might be associated with increased sliding pressure caused by the lateralized patellar tracking. Hence, the findings of this study also implied the influence of mechanical status on the development of trochlear dysplasia.
This study had some limitations. Firstly, this study was a retrospective study. The patients and their matched controls in this study were selected from consecutive cases admitted to our hospital in a limited period. To diminish potential selection bias, additional independent studies were required to validate the findings of this study. Secondly, this study evaluated the trochlear dysplasia on the proximal part where the patella engaged with the trochlear sulcus at the beginning of knee flexion. Biedert et al. had reported the characteristics of trochlear dysplasia by separating the trochlear into the proximal and distal parts . Further researches on the characteristics of the entire trochlear were needed to reveal the pathological changes of the trochlear. Lastly, although this study had detected significant results, based on the average femoral condyle width of 70 mm, medialization of the trochlear groove in these patients was only approximately 3 mm compared with that of the normal trochlear. The results cannot fully explain the increased TT-TG distance in patients with recurrent patellar instability, so a detailed evaluation is needed before surgical treatments.
In conclusion, this study demonstrated that dysplasia of trochlear morphology was related to the reduction of medial trochlear width and increase in lateral trochlear width. The medialization of trochlear groove was significantly correlated with the increased width of lateral trochlea.