A total of 75 knees (54 patients) with recurrent LPD and 75 knees (70 patients with similar age, gender and body mass index) with no medical history of knee joint were included in this study retrospectively.
Inclusion criteria: ①Recurrent LPD was diagnosed by two senior doctors in charge of joint and sports medicine department according to the patient's history, physical examination and MRI images. ②All patients had no previous experience in rehabilitation department or had any special training related to strengthening quadriceps muscle force. ③MRI images can be searched within 10 days after recurrent LPD.
Exclusion criteria: ①Patients with primary patellar dislocation. ②Traumatic patellar dislocations that occurred as a result of direct trauma to the medial patella or a fall onto the knee joint with concomitant patellar dislocation. ③Patients with any preexisting knee disorders, any prior knee surgery history, fractures of the distal femur or tibial head, and multi-ligament knee joint injury. ④Any patient with a history of neuromuscular disease (e.g., polio). ⑤Patients with obvious joint effusion.
Sagittal, coronal, and transverse MR images were obtained in all patients. Two experienced orthopaedic surgeons measured the following five parameters related to VMO (elevation on sagittal plane and coronal plane, cross-sectional area ratio, craniocaudal extent, muscle-fiber angulation,) and two parameters of patella tilt (patella tilt angle, bisect offset ratio) in both groups. And the type of femoral trochlear dysplasia in each patient was recorded according to Dejour et al (18) and Lippacher et al (19) classification system. Except for the calculation of cross-sectional area was completed by Image J freeware, other parameters were measured by picture archiving and communications system (PACS) workstation (Centricity, GE Healthcare, St. Gilles, United Kingdom). All parameters were repeatly measured with an interval of two weeks. MRI (Philips MR Systems Ingenia 3.0T, Andover, Massachusetts) protocols in our hospital were routine: all patients were in a supine position with knee naturally extended and quadriceps muscle fully relaxed.
The measurement of VMO elevation
The VMO elevation was measured on sagittal and coronal planes according to Zhang et al' s (20) measurement method. Briefly, the adductor tubercle could clearly be seen in the transverse slice was defined as the optimally measurable slice, indicated by the blue line (Fig. 1). In this transverse image, the corresponding sagittal and coronal planes were identified.
On the selected sagittal slice, the apex of the anterosuperior border of the bone cortex of the adductor tubercle was set as the starting point. The VMO elevation was defined as the shortest distance from the starting point extending obliquely to the inferior edge of the muscle belly (Fig. 1). On the selected coronal slice, the apex of the medial superior border of the adductor tubercle was set as the starting point. The VMO elevation was defined as the vertical distance from the starting point to the inferior margin of the VMO muscle (Fig. 1).
The measurement of VMO muscle-fiber angulation, craniocaudal extent of VMO and the cross-sectional area ratio of VMO
According to the method introduced by Balcarek et al's (21), VMO muscle-fiber angulation, craniocaudal extent of VMO and the cross-sectional area ratio of VMO were measured on MRI images. Firstly, the longitudinal axis of the patella was established in the central sagittal plane. In this sagittal image, the corresponding transverse slice located at the proximal patellar pole, and the adjacent slices located above and below this reference slice were identified. These transverse planes were used to measure the VMO cross-sectional area by manually drawing disarticulation contours around the muscle boundaries, and the whole thigh area at the corresponding level was also measured.
The cross-sectional area ratio of VMO was designed as the ratio between the cross-sectional area of VMO and whole thigh. Finally, the average value of the cross-sectional area ratio on three slices was calculated. The VMO muscle-fiber angulation, that is the angle between VMO muscle-fiber and the longitudinal axis of the femoral shaft, was measured on the sagittal plane. Furthermore, the lowest point of the VMO was located on this plane and the corresponding horizontal line was established in the sagittal plane centrally of the patella longitudinal axis. The craniocaudal extent of VMO was the vertical distance from this horizontal line to the proximal patellar pole.
The measurement of patella tilt angle and the Patella offset index
The transverse plane, that would allow visualization of the intact Roman arch and posterior femoral condyles was selected. The posterior condylar reference line was drawn tangent to the posterior femoral condyles. The patella tilt angle was formed by the maximal patella width line and the posterior femoral condyle line (Fig. 2).
According to the method of Christopher et al. (22)and Callaghan et al. (23), a line that passes through the deepest portion of the trochlear groove perpendicular to the posterior condylar reference line was drawn. The intersection of this line and the maximal patella width line was defined as point O. On transverse plane of the widest layer of the patella, the innermost point of patella was defined as point A and the outermost point as point B (Fig. 2). The percentage of OB / AB was defined as the bisect offset ratio.
SPSS 22.0 (IBM Corp. Released 2013. IBM SPSS Statistics for Windows. Armonk, NY: IBM Corp) was used to process the relevant data. All parameters were presented in the form of mean ± standard deviation. The comparison of continuous and categorical variables between the two groups were analyzed by independent-sample t test and Chi-square test, respectively. P <.05 was considered statistically significant. Moreover, the intraclass correlation coefficient (ICC) also was analyzed for duplicated measurements by two observers.