A new CT scan method for measuring the tibial tubercle trochlear groove distance in patellar instability
Introduction
Lateralisation of the tibial tuberosity can cause lateral tracking patella leading to potential patellar instability with patellar pain or to objective patellar instability [1], [2]. Surgery should be aimed at correcting the anatomical deficiency (i.e. patella alta, trochlea dysplasia, medial retinalular insufficiencies, muscular imbalance, or an increased tibial tuberosity (TT) trochlear groove (TG) distance). The key to successful treatment of both lateral tracking patellae and objective patellar instability is the proper identification of the anatomical deficiency. If tuberosity transfer is performed for ill-defined patellofemoral pain without anatomical abnormality, the intervention will lead to iatrogenic damage [2].
Imaging is needed to confirm the malalignment diagnosis and to accurately plan the operative medialisation. Conventional radiography is often the first imaging modality performed. It is accurate in evaluating numerous patellofemoral disorders but is not suitable for determining distal patellofemoral malalignment [3]. Computed tomography (CT) of the patellofemoral joint is an excellent technique for evaluating distal patellofemoral malalignment, and the distance between the middle of the tibial tuberosity and the bottom of the trochlear groove of the femur is a recognised measurement to express lateralisation of the tuberositas tibiae [4], [5], [6]. The tibial tuberosity trochlear groove can also be assessed on MRI using either cartilage or bony landmarks. When using bony landmarks an excellent intermethod (86%) reliability between CT and MRI is found [7].
If an increased tibial tuberosity trochlear groove distance is detected, a distal realignment procedure to correct the anatomical deficiency can be accurately planned based upon the quantitative CT tibial tuberosity trochlear groove measurement. However, in our clinical practice we have experienced some problems in interpreting the CT scans. Identification of the anatomical structures on the superimposed maximum intensity projection (MIP) can be difficult. This leads to a low intra- and inter-observer reliability. Previously, Saudan reported on this problem [8]. A reliable, CT based method to determine malalignment is needed to identify those patients who may benefit from surgical intervention. The aim of this study was to compare the reliability of two methods based on transverse CT images to measure the tibial tuberosity trochlear groove distance.
Section snippets
Methods
A cohort of 25 patients (50 knees) who were suspected of having either a lateral tracking patella or objective patellar instability were included in this study. All patients were suspected of having an increased tibial tuberosity trochlear groove distance. Thus this cohort reflected our clinical population. Some did indeed have an increased tibial tuberosity trochlear groove while others had a normal tibial tuberosity trochlear groove distance.
Interclass correlation
None of the 50 CT scans was rejected because of inadequate quality. Table 1 shows that the aggregate ICC for the double image technique is 0.970. The aggregate ICC for the new single image technique is 0.980. Both methods show extremely little variation in the inter- and intra-observer variability. Furthermore there is no difference in variability between the four observers as the CI95 between the lowest and highest individual observer ICC falls within the CI95 from the pooled data from all
Discussion
The principle findings in this study were that both the double image and the new single image technique tibial tuberosity trochlear groove CT measurement techniques provide a reliable method of determining tuberositas lateralisation, but that the measurement error is lower for the single image technique. The ICC for the double image technique is 0.970; the ICC for the single image technique is 0.980. We think that this high ICC is due to the thorough instruction that all observers received for
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