ULTRASOUND BASED VERSUS POINTER PALPATION METHOD IN THA NAVIGATION – A COMPARATIVE STUDY

Abstract

In THA navigation systems mostly the anterior pelvic plane (APP) is used as reference for cup orientation. For the femoral torsion the sagittal plane of the leg in 90° knee flexion is used as reference. The common procedure is associated with a pointer based palpation of anatomical landmarks. But sometimes it shows difficulties especially in obese patients. The purpose of the current prospective study was to evaluate the THA navigation based on palpation of the APP and the femoral sagittal plane using a novel intra-operative ultrasound device in comparison to standard palpation technique.

Methods: In 2006 the data of 50 patients with navigated THA were analyzed. An image free computer-assisted navigation system (OrthoPilot, B. Braun Aesculap, Germany) was used together with an integrated ultrasound device (probe: 80mm, 5–8 MHz, Telemed, Latvia). First, the registration of bony landmarks, i.e. anterior iliac spines and symphysis for APP as well as patella and tibial pylon for the femoral sagittal axis was performed using the conventional pointer method (PO). Then registration was repeated using the ultrasound device (US). The landmarks of the APP and the dorsal condylar line to define the femoral torsion alignment were recorded. The intra-operative cup orientation and the resulting range of motion was monitored. After cup placement in the “safe zone” described by Lewinnek the femoral stem was rasped monitoring the predicted final ROM, leg length and offset. The navigation software provides simultaneously data for for both, pointer and ultrasound palpation. Cup anteversion, inclination, the antetorsion angle of the rasp as well as the predicted amount of impingement free internal (IR) and external rotation (ER) of the leg are shown. At the end of operation ROM was checked clinically. Plain standardized post-operative X-rays were evaluated for inclination and anteversion angles according to Pradhan.

Results: The calculated mean difference between US registration and PO palpation was 2,2° (range -1,7 – 5.9°) for the cup inclination and 8,7° (range 1,5° – 16°) for anteversion. The impingement free total ROM of the femur as predicted by the Orthopilot, i.e. 109° for PO and 103° for US registration matched well with the clinical data. The clinical findings confirmed the US registration based ratio of mean external (ER 62°) to internal rotation (IR 41°). However, the predicted mean IR as calculated for PO palpation, i.e. 31°, was reduced by a mean of 9° in favor of 13° of more ER (76°).

Conclusion: The results show a higher coincidence of US based registration with the clinical findings. The difference is due to a registration at the bone surface with the US method, while PO palpation cannot overcome a undefined thickness of soft tissue layer, especially in front of the symphysis in obese patients.