Study design
This is a case control study that uses a MRI model, previously described to study ACL injuries[6–9], to study the sagittal plane articulation of the tibiofemoral joint of the contralateral knee of subjects with PCL deficiency.
Subject selection
Twenty subjects participated in the study. 10 subjects with no history of knee complaints and normal clinical examination were used as controls. The control subjects were aged between 26 and 39 years. There were 5 females and 5 male subjects. 10 subjects with unilateral PCL injuries were recruited for the study. Isolated PCL injury was diagnosed on clinical examination and MRI. On clinical examination PCL injury was suggested by posterior sag and posterior draw test. The dial test was used to exclude subjects with concomitant posterolateral corner injuries. Subjects were excluded if there were any contraindications to MRI, may have been pregnant, or if they were over 180 cm tall (to permit knee flexion in the MRI tunnel). 5 subjects had an acute PCL injury and 5 subjects had a chronic PCL injury. An acute PCL injury was defined as the persistence of bone bruising on diagnostic MRI at the time the study was undertaken. At the time the study was undertaken, the time since injury ranged from 2 to 9 weeks. There were 2 females and 3 males. The age of these subjects ranged from 18 to 27 years. 4 of the subjects had a knee brace locked in extension and had not began physiotherapy exercises and one subject was 3 weeks from removal of the brace and had begun quadriceps strengthening exercises. All subjects had an effusion on examination, decreased range of motion compared to the contralateral knee but no patellofemoral crepitus. Four subjects sustained the injury through sports (one from netball, one from soccer and two from rugby) and one subject outside of sport (sustained injury whilst falling from a two metre height). The age of the subjects with a chronic PCL injury ranged from 39 to 47 years. There were 3 females and 2 males. PCL injuries were sustained from a time period of 5 to 21 years. No subject had bone bruising on MRI at the time of the study and no effusion on clinical examination. All subjects complained of no symptoms from their knee during activities of daily living. All subjects were able to bicycle with their knee injury. All subjects in the chronic PCL group sustained the injury whilst playing sport (two from rugby and three from netball). Clinically the chronic PCL injury group were examined for evidence of degenerative joint disease. None of the subjects demonstrated joint line tenderness or had reduced range of motion compared to the contralateral side. However, all of the subjects had patellofemoral joint crepitus. All subjects provided informed consent. Ethics approval for the study was obtained from the Department of Health and university human research ethics committees.
MRI imaging procedure
Subjects performed a supine leg press between 0 and 90 degrees on a wooden frame with a sliding footplate fitted to the MRI couch. The leg press was weighted by a 150 N load via a rope and pulley to resist leg extension and thereby simulate a weight bearing squat (Figure 1). Elastic straps stabilised the thighs, feet and ankles. Imaging of both knees simultaneously was performed. Parasagittal images perpendicular to the tibial plateau were generated through each knee
Tibiofemoral contact point measurement
The position of the tibiofemoral contact (TFC) with the tibial plateau was recorded as the distance from the posterior tibial cortex to the point of the TFC of the medial and lateral femoral condyle (Figure 2). Where contact occurred over a wide area, the area centroid was used. To account for variation in the size of subjects, cortex to contact distance measurements were normalised to a tibial plateau size of 50 mm. The mean anterior-posterior diameter of the medial tibial plateau was 48 +/- 5.4 mm, and the lateral tibial plateau was 41 +/- 2.4 mm.
Flexion facet centre measurement
The position of the flexion facet centre (FFC) over the tibial plateau was located by using a three stage measurement technique with a computer assisted design program (Figure 3). First, the FFC was identified by fitting a circle to define the flexion arc of the posterior condyle. This involved using an arc function to identify 3 points on the posterior aspect of the femur which could then be incorporated into a circle of bit fit. Second, the tibial plateau was defined by a line from the posterior tibial cortex, parallel to the tibial plateau. Lastly, a line was drawn through the FFC perpendicular to the tibial plateau line to measure the distance from the posterior tibial cortex to the intersection of the perpendicular line.
Precision
The precision of both methods of measurement was tested by repeating measurement from the original scanned images on two occasions at least 24 hours apart. The precision of mapping the contact points for the medial and lateral compartments was very high with intra class correlation 0.95 (99% confidence interval was 0.92 0.96). The precision of measuring of the FFC was also very high with intra class correlation of 0.93(95% confidence interval was 0.88-0.93). The greatest difference observed between the repeated measurements was 0.7 mm for the mapping the TFC point and 0.9 mm for mapping the FFC.
Statistical analysis
Statistical analysis was carried out using statistiXL version 1.8 for Microsoft Excel. A two-way repeated measures analysis of variance with Tukey and Scheffe post hoc tests were used to compare the TFC points and FFC positions between the healthy and the PCL deficient groups. A p value of less than 0.05 was regarded as statistically significant.