PATELLA DISPLACEMENT ASSESSMENT

landmarks and re-marked using a new piece of tape. The two measurements are recorded and averaged to give the patient’s lateral patella displacement.

Figure 1: Measurement of patella displacement

showed that a lack of engagement between the patella and the trochlear groove results in a greater than normal patella displacement (3). The tibia must also be in the anatomical position, as it has been shown that tibial rotation influences the motion and position of the patella (7). Without this correct positioning, it can cause misleading results to be collated. This is followed by applying a strip of hypoallergenic tape across these landmarks. The therapist then marks the distance from the medial and lateral epicondyles of the femur and the centre of the patella on to the hypoallergenic tape, as in Fig. 1, to produce a measurement similar to that shown in Fig. 2. To establish the amount of displacement, the equation in Fig. 3 can be used. A positive value indicates a laterally displaced patella position, while a negative value indicates a medially displaced patella position. In the majority of cases, the medial distance (A) is greater than the lateral distance (B), indicating a laterally displaced “normal” patella position. The process is repeated from the initial stage of palpating the anatomical

THE SALFORD STUDY The Salford study found that the method used to measure the lateral patella displacement using ultrasound imagery complemented that of the tape measurements. The patient was positioned lying supine and in a position of 20° of knee flexion, with the quadriceps muscle group relaxed. Comparative pairs of ultrasound images were captured using real-time ultrasonography (in this case, an AU5 Advanced Ultrasonography, Esaote Biomedica, Milan, Italy). Images were obtained by placing the ultrasound probe (7.5 MHz linear array probe) in the frontal plane from the lateral femoral epicondyle in line with the apex of the patella, perpendicular to the dermal surface, with water-soluble transmission gel placed over the scan head. An example of the images that were produced by this method can be seen in Fig. 4.

Ultrasound images were analysed

offline with image analysis software ImageJ (National Institutes of Health, http://rsb.info.nih.gov/ij/index.html) to provide measurements of the lateral position of the patella in relation to the femoral epicondyles (B). As with the tape-measurement method, two distances were recorded and averaged to give the patient’s lateral patella displacement. This measurement produces a more accurate but comparative value of displacement

Video 1: Video showing patella displacement measurement technique (online) epicondyle A

centre of patella

lateral B Figure 2: Measurement of patella displacement A - B = patella displacement Figure 3: Patella displacement equation epicondyle

Figure 4: Use of ultrasound to measure patella displacement

to that produced through the tape method.

CLINICAL TRIAL RESULTS When these techniques were implemented and measured in a clinical setting at the University of Salford, under the supervision of Mr Lee Herrington, they produced statistically reliable and comparable results. This trial consisted of 20 participants (10 male, 10 female; mean age 20.95 ± 14.05 years). In a position of 20° of knee flexion, with the tibia in the anatomical position, the centre of a “normal” patella was found to be positioned 4.986 ± 0.162 mm lateral to the midpoint between the femoral epicondyles. This value is in line with in vivo and biomechanical models of patella tracking as well as previous research conducted by Herrington (5), which showed a lateral patella displacement using a similar methodology of 5.7 ± 0.585 mm. Using the tape measurement method described by McConnell (6), the mean lateral displacement of the patella was 4.905 ± 0.305 mm. Males have been found to have a lateral displacement of 5.27 ± 3.07 mm when measured using the tape methodology and females 4.54 ± 2.44 mm. The results

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