Anatomical large femoral heads: early complications and mid term results - Статья

УДК 616.728.2-089.843(045)=111 Tel-Aviv Medical Center, Sackler Faculty of Medicine, Tel-Aviv University. Israel NYU Langone Medical Center Hospital For Joint Disease Anatomical large femoral heads: early complications and mid term results A. Grundshtein, O. Chechik, New York. USA One of the major challenges in performing a Total Hip Arthroplasty is restoration of native hip anatomy for better surgical results. Complications, such as dislocations (occur in about 3 % of cases), postoperative fractures (1 % of cases), leg length inequality (symptomatic in up to 27 % in some series), abductor mechanism weakness and device failure can be attributed in part to inaccurate restoration of native hip anatomy [1-3]. Human hip has great anatomical variability regarding size, version, neck length, valgus/varus position and offset [4]. Each prosthesis design has a different degree of freedom in fitting a specific anatomy - different sizes, different neck lengths, standard vs. high offset design. Modularity was utilized to conform further characteristics of the femur, but there were reports of failure at the interface between the different modular parts [5-7]. Methods to increase accuracy of cuts and implant positioning, such as pre-op planning (digital/ manual templating), intra-operative measurements and navigational techniques are combined as well as surgical technique (e.g. level of osteotomy and version determination by angle of insertion into the femur) all play a crucial role [8]. Fig. 1. General overview of the eccentric displacement mechanism. The off-center positioning of the head over the taper allowed by the large head size results in different anatomical changes for each direction: anterior position (a), inferior position (b), posterior position (c), superior position (d) Versatility of the anatomical femoral mega-head and the resulting anatomical changes Head Size Direction of displacement Maximal displacement (mm) Resulting change in joint anatomy per 1mm Range Up to 45 mm Anterior/ posterior 4 1.3-1.5° change in version H L/i о Superior 7 0.7 mm in offset ± 5.6 mm Inferior 11 46 mm and larger Anterior/ posterior 8 1.3-1.5° change in version ± 10° Superior 11 0.7 mm in offset ± 9.1 mm Inferior 15 Note. Different head sizes offer different ranges for implant versatility, which also differ in the range for different directions. The distance of off-center displacement is translated into changes in the resulting anatomy. However those options do not always suffice and may lead to a sub-optimal result due to patients specific anatomy. Attention to each of the anatomical variables is essential. The global offset Is the sum of the femoral offset and the lateralization of the hips center of rotation, and its preservation allows restoration of the abducting musculature and the lever arm, increased stability and range of motion and decreased wear [9]. Version refers to the orientation of the neck in reference to the coronal plane of the femur and is denoted as ante, neutral or retroversion and its restoration is essential in achieving stability [10]. Leg length inequality over 1 cm can cause patient discomfort and dissatisfaction, and lengthening of over 2 cm may increase the risk of sciatic nerve palsy [11, 12]. Short neck lengths requiring low femoral neck osteotomies can cause early loosening and fractures. The anatomical femoral mega-head is a novel approach (DePuy Synthes, Warsaw, Indiana, USA), which enables a surgeon to replicate normal anatomy of each hip and increase versatility during surgery. Owing to large head size, the head can be placed eccentrically over the taper in any direction, each resulting in a specific change in biomechanics (Fig. 1). Displacing the head to the anterior or posterior direction causes change in ante or retro-version, with displacement limited by the head size (range ±10 degrees). The anatomical large head can also be set with greater neck length. Digital preoperative planning on Anteroposterior and Axial radiographs is done in order to determine the desired position of the head for optimal anatomical restoration, with intra operative measurements and modifications to achieve the best result. Moving the taper from the center of the head is made possible by the large head size. The use of metal on metal implants offered a few theoretical advantages - elimination of the Polyethylene liner wear, increased stability due to increased head size and range of motion [13-15]. However, as clinical experience grew, new problems appeared. Elevated blood metal ion levels, local tissue responses to metal debris, and unacceptable rates of failure, eventually leading to discontinuing of further use for several implants [16-18], as well as the evolving field of taper neck trunionosis [19, 20]. The possible advantage of this system is its versatility and ability to restore precisely the anatomy of the native hip, especially addressing extreme variations (varus, valgus and extremely short/long nec