Is determination of the hip rotation center during computer-assisted surgery influenced by the pinless femoral sensor attachment method? An in vitro preliminary study comparing osseous pin fixation.

INTRODUCTION: Computer-assisted surgery improves the positioning of hip prostheses but requires use of transosseous pins requiring a complementary approach exposing the patient to rare but at times serious complications. The use of sensor arrays attached to the skin could advantageously replace pins provided that comparable results are obtained, but their validity has not yet been assessed. We conducted a prospective in vitro study to: measure the possible error of a cutaneous versus transosseous fixation to determine the hip rotation center (HRC) position and determine the inter- and intraobserver reproducibility of the cutaneous versus the transosseous fixation. HYPOTHESIS: Use of cutaneous sensor arrays while recording the HRC is sufficiently reliable for its calculation algorithm to provide measurement accuracy within 5mm. MATERIALS AND METHODS: A rigid array attached with either a silicone strap or an adhesive were compared to a transosseous array. Four series of 96 HRC measurements were collected by four operators on two cadavers, half with an array attached with a strap and half with an adhesive. The results were compared to those obtained by a sensor attached with transosseous pins. RESULTS: On condition that the hip-knee is mobilized in extension, a sensor array attached with an adhesive gives results with comparable accuracy (standard deviation [SD]: 2.89mm [1.9-4.8]) to the results obtained with a transosseous fixation (SD: 1.2mm [0.9-1.6]), with no significant inter- or intraobserver variation (0.97

Total knee arthroplasty three-dimensional kinematic estimation prevision. From a two-dimensional fluoroscopy acquired dynamic model.

INTRODUCTION: To determine six-degree of freedom of total knee arthroplasty kinematics (TKA), optimized matching algorithms for single fluoroscopic image system may be used. Theoretical accuracy of these systems was reported. Nevertheless, all reports were done under idealized laboratory experimental conditions. The aim of this study was to evaluate the "true" accuracy of a flat panel single plane video-fluoroscopy system based on computed-assisted design (CAD) model matching and compare it to TKA kinematics obtained from optoelectronic measurements as gold standard. HYPOTHESIS: The estimation of the error produced by 2D/3D fluoroscopic registration in daily practice is misjudged in most available laboratory reports. MATERIAL AND METHODS: The experimental set-up used a TKA implanted into femoral and tibial cadaver bones. Thirty flexions were simultaneously registered using single plane fluoroscopy and an active optical tracking system. Kinematics registered were compared using the root mean square error (RMS), the concordance correlation coefficient and Bland & Altman plot analysis. RESULTS: The mean range of motion of flexion during the experiment was 106°. The respective RMS for flexion, varus-valgus and internal-external rotation were 0.68, 0.67 and 1.02°. The respective RMS for antero-posterior, medio-lateral and proximo-distal displacement were 1.3, 2.4 and 1.06 mm. Extreme values of the measured error concerning medio-lateral displacement were -5.4 and 22,1mm. DISCUSSIONS: Analysis found some outliners in all degree of freedom with a systematic error and larger standard deviation than already published data. One should make sure that during the experiment the motion of interest is in the in-plane direction. Moreover, this study brings out the true threshold detection of this type of analysis.

[Development and validation of a dynamic model of the knee]. / Elaboration et validation d'un modèle numérique de genou.

The authors report the methodology of the construction of a multibody model of the knee and the validation of the kinematics of the modelled knee. The construction of the model includes: the rigid bodies represented by osseous components (femur, tibia, fibula, patella), the ligamentous structures (collateral ligaments, patellar ligament, cruciates ligaments), the muscular part represented by the quadriceps. Morphological data were acquired through 3D CT scans for the bones and a biometrical study of the ligaments (insertions, orientation, length, section). Ligament biomechanics was modelled as bilinear springs (in compression the tightness is null; in traction it is a function of length, section and Young modulus of elasticity). The quadriceps was modelled as a sliding channel with a translatory servocommand. Contacts at the interfaces (femur/patella; femur/tibia) were evaluated according to the index of penetration (distance D) between two bodies where effort was: Dx10(5) N/mm(2)). The model was tested simulating a symmetrical kneeling (800 N body weight) and required a ground link modelled as a ball and socket joint. The model was developed under ADAMS software. The validation of the kinematics of the modelled knee was provided according to the data of Wilson et al. who have shown that (i) in normal knees, internal/external rotation, abduction/adduction and all three components of translation are coupled to flexion angle both in passive flexion and extension; (ii) the tibia rotates internally as the knee is flexed. The consistency of the coupled motions support the model's premise that passive knee motion is guided by isometric fascicles in anterior and posterior cruciates, by the medial collateral ligament and by articular contact in the medial and lateral compartments. The main curves (internal/external rotations; posterior/anterior translation) of the model conforms with the framework of Wilson.

The anatomic basis for the concept of lateralized femoral stems: a frontal plane radiographic study of the proximal femur.

We determined the range of sizes for a system of monoblock femoral prostheses that would provide adequate (a term defined in the text) fill in the frontal plane and restore femoral offset and leg length. We performed an anatomic study, based on measurements in 200 anteroposterior pelvic radiographs. If diaphyseal filling implants are to be used, 9 sizes are sufficient to obtain excellent canal filling and restoration of femoral offset in >80% of cases, assuming that the level of neck osteotomy can vary over a 1-cm range. When using metaphyseal filling implants, only a limited adjustment can be obtained from the level of neck osteotomy. A system limited to 8 sizes approximates the anatomy of the femoral canal with satisfactory precision in 73% of cases. If such a system is provided with only a single neck shaft angle for each stem size, it does not allow restoration of the biomechanical center of the hip in >67% of cases. A system of 8 sizes of 1 neck/shaft angle and a 22-mm modular head restores the anatomy in only 49% of cases. Approximating the frontal anatomy of 85% of femora with an implant filling the metaphysis requires at least 15 sizes distributed in 3 metaphyseal configurations, each supplied with 2 different neck shalt angles.