Purpose Patella alta is a risk factor for patellofemoral pain and instability. Several measurement methods and imaging modalities are in use to measure patellar height. The first aim of this study was to determine the intra-and interrater reliability of different patellar height measurement methods on conventional radiography (CR), CT and MRI. The second aim was to examine the applicability of patellar height measurement methods originally designed for CR on CT and MRI. Methods Forty-eight patients who were treated for patellar instability were included. All patients had undergone a pre-operative conventional radiograph, CT scan and MRI. Five methods for measuring patellar height were performed on radiographs, CT and MRI by four observers. For each measurement, the intra-and interrater reliability was determined by calculating the intra-class correlation coefficient (ICC). A Bland-Altman analysis was performed for measurements with an ICC ≥ 0.70. ResultsThe Insall-Salvati (IS) ratio was the only measurement that showed good intra-and inter-observer reliability on CR, CT and MRI. The intra-and inter-observer reliability of the patellotrochlear index (PTI) for MRI was good to excellent for all observers. The IS ratio showed a moderate to good reliability for comparison of all three imaging modalities with the best agreement between radiography and MRI. The other patellar height measurements showed only poor to moderate inter-method agreement. Conclusion In this study, the Insall-Salvati ratio shows better intra-and inter-observer reliability than the Blackburne-Peel ratio, the Caton-Deschamps ratio and the modified Insall-Salvati ratio on all imaging modalities. Radiography and CT seem to have better reliability than MRI. The patellotrochlear index, however, shows good inter-and intra-observer reliability on MRI. Only for the IS method was there acceptable agreement between CR and MRI. This means that the established Insall-Salvati normal values could be used for MRI as well. This study shows that the most reliable method to measure patella height is the Insall-Salvati ratio measured on conventional radiographs or the patellotrochlear index on MRI. Level of evidence Level II diagnostic.

A 63-year-old man reported numbness in his left first, second and third digits for 2 years. Initially, he found that 'shaking out' his left hand helped, but this benefit did not last. He underwent decompression of left-sided carpal tunnel syndrome 2 years earlier and his symptoms worsened after surgery, with more sensory loss and pain. He worked as a carpenter, and had injured his left thumb with nails on multiple occasions without feeling pain. On examination, he had an enlarged left thumb (figure 1), though he appeared unaware of this; his other digits were normal in size. The median nerve was palpable at the wrist, but Tinel's test was negative. There was a mild weakness of abductor pollicis brevis and opponens brevis, with reduced sensation over the fingertips of the lateral three digits and palm.Electrophysiological testing confirmed median nerve entrapment at the carpal tunnel, with absent sensory potentials at the third and fourth digits, with prolonged distal motor latency (5.6 ms), and decreased motor compound muscle action potential at the carpal tunnel (compound muscle action potential 2.7 mV). Proximal to the carpal tunnel inlet, the conduction velocities were normal (47.3 m/s at the forearm and 46.7 m/s at the upper arm). Electrodiagnostic testing of the ulnar nerve was normal. We did not perform needle electromyography. Ultrasound scan showed a hypertrophic enlarged median nerve along the entire tract in the left forearm (figure 2), but the main enlargement was at the wrist, where the cross-sectional area was 136 mm 2 (normal <12). There were no abnormalities in the ulnar and radial nerve, and plexus brachialis. A T1-weighted MR scan with gadolinium of the left arm showed an enlarged median nerve with lipomatous infiltration (figure 3), which is quite specific for macrodystrophia lipomatosa.The differential diagnosis of hypertrophied nerves has been discussed previously. 1 The pathophysiology that causes nerve hypertrophy can be very diverse, and includes entrapment neuropathies, leprosy, hereditary neuropathies and inflammatory neuropathies. Important sonographical features of nerve hypertrophy are diffuse or focal thickening and Figure 1 The patient's enlarged left thumb, with other digits of normal size.Figure 2 Ultrasound scan of the median nerve at the wrist, showing hypertrophic enlarged nerve with characteristic honeycomb structure.