Milad Masjedi

Why large-head metal-on-metal hip replacements are painful: THE ANATOMICAL BASIS OF PSOAS IMPINGEMENT ON THE FEMORAL HEAD-NECK JUNCTION

Large-head metal-on-metal total hip replacement has a failure rate of almost 8% at five years, three times the revision rate of conventional hip replacement. Unexplained pain remains a feature of this type of arthroplasty. All designs of the femoral component of large-head metal-on-metal total hip replacements share a unique characteristic: a subtended angle of 120° defining the proportion of a sphere that the head represents. Using MRI, we measured the contact area of the iliopsoas tendon on the femoral head in sagittal reconstruction of 20 hips of patients with symptomatic femoroacetabular impingement. We also measured the articular extent of the femoral head on 40 normal hips and ten with cam-type deformities. Finally, we performed virtual hip resurfacing on normal and cam-type hips, avoiding overhang of the metal rim inferomedially. The articular surface of the femoral head has a subtended angle of 120° anteriorly and posteriorly, but only 100° medially. Virtual surgery in a normally shaped femoral head showed a 20° skirt of metal protruding medially where iliopsoas articulates. The excessive extent of the large-diameter femoral components may cause iliopsoas impingement independently of the acetabular component. This may be the cause of postoperative pain with these implants.

Robotic Assistance Enables Inexperienced Surgeons to Perform Unicompartmental Knee Arthroplasties on Dry Bone Models with Accuracy Superior to Conventional Methods

Robotic systems have been shown to improve unicompartmental knee arthroplasty (UKA) component placement accuracy compared to conventional methods when used by experienced surgeons. We aimed to determine whether inexperienced UKA surgeons can position components accurately using robotic assistance when compared to conventional methods and to demonstrate the effect repetition has on accuracy. Sixteen surgeons were randomised to an active constraint robot or conventional group performing three UKAs over three weeks. Implanted component positions and orientations were compared to planned component positions in six degrees of freedom for both femoral and tibial components. Mean procedure time decreased for both robot (37.5 mins to 25.7 mins) (P = 0.002) and conventional (33.8 mins to 21.0 mins) (P = 0.002) groups by attempt three indicating the presence of a learning curve; however, neither group demonstrated changes in accuracy. Mean compound rotational and translational errors were lower in the robot group compared to the conventional group for both components at all attempts for which rotational error differences were significant at every attempt. The conventional groups positioning remained inaccurate even with repeated attempts although procedure time improved. In comparison, by limiting inaccuracies inherent in conventional equipment, robotic assistance enabled surgeons to achieve precision and accuracy when positioning UKA components irrespective of their experience.

Cam type femoro-acetabular impingement: quantifying the diagnosis using three dimensional head-neck ratios

ObjectiveCam hips are commonly quantified using the two-dimensional α angle. The accuracy of this measurement may be affected by patient position and the technician’s experience. In this paper, we describe a method of measurement that provides a quantitative definition of cam hips based upon three-dimensional computed tomography (CT) images.Materials and methodsCT scans of 47 (24 cam, 23 normal) femurs were segmented. A sphere was fitted to the articulating surface of the femoral head, the radius (r) recorded, and the femoral neck axis obtained. The cross sectional area at four locations spanning the head neck junction (r/4, r/2, 3r/4 and r), perpendicular to the neck axis, was measured. The ratios (Neck/Head) between the areas at each cut relative to the surface area at the head centre were calculated and aggregated.ResultsNormal and cam hips were significantly different: the sum of the head-neck ratios (HNRs) of the cam hips were always smaller than normal hips (p < 0.01). A cut off point of 2.55 with no overlap was found between the two groups, with HNRs larger than this being cam hips, and smaller being normal ones.ConclusionOwing to its sensitivity and repeatability, the method could be used to confirm or refute the clinical diagnosis of a cam hip. Furthermore it can be used as a tool to measure the outcome of cam surgery.

Unicompartmental knee arthroplasties: Robot vs. patient specific instrumentation

BACKGROUND The technical reliability demonstrated by semi active robots in implant placement could render unicompartmental knee arthroplasties (UKAs) more favourable than they are currently. The relatively untested method using patient specific instrumentation (PSI), however, has the potential to match the accuracy produced by robots but without the barriers that have prevented them from being used more widely in clinical practice, namely operative time. Therefore this study took a step towards comparing the accuracy and time taken between the two technologies. METHODS Thirty-six UKAs were carried out on identical knee models, 12 with the Sculptor, 12 with PSI and 12 conventionally under timed conditions. Implant placement in these knees was then judged against that in a pre-operative plan. RESULTS Tibial implant orientations and femoral implant positions and orientations were significantly more accurate in the PSI group with mean errors of 6°, 2 mm and 4° respectively, than the conventional group which had means of 9°, 4 mm and 10°. There was no significant difference between the robot and PSI generally except in tibial implant orientation (mean robotic error 3°) and tibial implant position did not vary significantly across all three groups. It was also found that use of PSI and conventional methods took half the time taken by the robot (p<0.001). CONCLUSIONS With further development, PSI can match and possibly surpass the accuracy of the robot, as it does with the conventional method, and achieve planned surgery in less time. CLINICAL RELEVANCE This work sets the foundation for clinical trials involving PSI.

Musculoskeletal shoulder models: A technical review and proposals for research foci

Musculoskeletal shoulder models allow non-invasive prediction of parameters that cannot be measured, particularly the loading applied to morphological structures and neurological control. This insight improves treatment and avoidance of pathology and performance evaluation and optimisation. A lack of appropriate validation and knowledge of model parameters’ accuracy may cause reduced clinical success for these models. Instrumented implants have recently been used to validate musculoskeletal models, adding important information to the literature. This development along with increasing prevalence of shoulder models necessitates a fresh review of available models and their utility. The practical uses of models are described. Accuracy of model inputs, modelling techniques and model sensitivity is the main technical review undertaken. Collection and comparison of these parameters are vital to understanding disagreement between model outputs. Trends in shoulder modelling are highlighted: validation through instrumented prostheses, increasing openness and strictly constrained, optimised, measured kinematics. Future directions are recommended: validation through focus on model sub-sections, increased subject specificity with imaging techniques determining muscle and body segment parameters and through different scaling and kinematics optimisation approaches.

Mathematical representation of the normal proximal human femur: application in planning of cam hip surgery.

Precise modelling of the proximal femur can be used for detecting and planning corrective surgery for subjects with deformed femurs using robotic technology or navigation systems. In this study, the proximal femoral geometry has been modelled mathematically. It is hypothesised that it is possible to fit a quadratic surface or combinations of them onto different bone surfaces with a relatively good fit. Forty-six computed tomography datasets of normal proximal femora were segmented. A least-squares fitting algorithm was used to fit a quadratic surface on the femoral head and neck such that the sum of distances between a set of points on the femoral neck and the quadratic surface was minimised. Furthermore, the position of the head–neck articular margin was also measured. The femoral neck was found to be represented as a good fit to a hyperboloid with an average root mean-squared error of 1.0 ± 0.13 mm while the shape of the femoral articular margin was a reproducible sinusoidal wave form with two peaks. The mathematical description in this study can be used for planning corrective surgery for subjects with cam-type femoroacetabular impingement.

A method of assessing the severity of cam type femoro-acetabular impingement in three dimensions

Femoroacetabular impingement is caused by abnormal morphology of either the femur or acetabulum or both. Diagnostic criteria currently include an α angle of over 50° on a lateral radiograph. In this study, CT scans of symptomatic hips (n = 37) were compared with normal hips (n = 34) obtained from CT colonoscopy procedures. The femoral head described in terms of a three dimensional (3D) α angle and a 3D head neck margin (epiphysis) angle ‘3Dμ’ using a semi-automated algorithm. In normal hips 70% have a maximum 3Dα angle of more than 50° at some point around their femoral head (mean 53° ± 5°, range 42° - 64°), while in cam hips, it was significantly larger (mean 69° ± 10°, range 54° – 94°, p<0.001). The 3Dμ also varied significantly and had a reverse relationship to that of the α angle: cam hips have an articular extent that crossed over spherical limit of the hip joint (mean minimum 41° ± 7°) while the articular margin of normal hips always remained within the spherical limit (mean minimum 49° ± 6°). This semi-automated algorithm provides an objective measure of the femoral head in health and disease. It can reliably distinguish cam hips from normal, enabling cam hips to have their cam quantified and their surgery planned objectively.

A local reference frame for describing the proximal human femur: application in clinical settings

ObjectiveThe conventional reference frame for the femur has limited relevance for the planning of hip surgery as the femoral neck axis, a crucial reference for surgeons, has to be independently derived. The purpose of this study is to develop and validate a reliable frame of reference for the proximal femur that can be applied in clinical settings.Materials and methodsTen three-dimensional models of femurs were obtained. An iterative method was developed to find the femoral neck axis (X-axis). A second axis was also created from the lesser trochanter to the piriformis fossa (LTPF). The origin was defined as the femoral head centre. The cross product of the neck and LTPF axes provided the Z-axis and the third axis (Y-axis) was perpendicular to the other two. Intra-/inter-investigator reliability was assessed on the ten femur models; ten times by one investigator and twice by three investigators respectively. The results were then compared with the conventional reference frame using landmarks on the distal femur.ResultsThe femoral neck and LTPF axes had mean intra-/inter-investigator angle differences of 0.5° (SD 0.4°) and 0.7° (SD 0.5°), and 0.8° (SD 0.5°) and 0.9° (SD 0.6°) respectively while the variations of the X-, Y- and Z- axes were SD 0.6°, 0.7° and 0.5°.ConclusionsA reliable method of obtaining the three-dimensional proximal femoral frame was developed, using the femoral neck axis, with greater relevance to clinical settings, preoperative planning and accurate assessment of procedures post-operatively.

The knee adduction angle of the osteo-arthritic knee: a comparison of 3D supine, static and dynamic alignment.

BACKGROUND End-stage knee osteoarthritis (OA) commonly results in knee arthroplasty. Three dimensional (3D) supine imaging is often used for pre-operative planning to optimise post-operative knee adduction angles (KAA). However, supine imaging may not represent loaded knee alignment. The aim of this study was to investigate differences in knee alignment under supine, static and dynamic conditions in healthy subjects and subjects with knee OA. METHODS Nine healthy subjects and 15 subjects with end-stage knee OA were recruited. All subjects underwent supine imaging and motion capture during gait. KAAs were calculated from supine images (SUPINE), upright standing (STATIC) and at the first peak ground reaction force during gait (DYNAMIC), and were compared. RESULTS KAAs were significantly higher (more varus) during gait compared with static (loaded and unloaded) in healthy subjects (p<0.01) but not in subjects with knee OA. There was a good correlation between SUPINE and DYNAMIC for both healthy and OA subjects (R(2)>0.58), with differences in the two relationships; healthy knees had a higher KAA during gait for any given KAA in the supine position, whereas OA knees that were valgus in imaging became more valgus during gait, and the opposite occurred for varus knees. CONCLUSIONS Factors that may contribute to the noted differences between healthy and OA subjects include morphological changes in the joint as a result of OA, and gait compensation strategies in people with end-stage OA. Dynamic 3D motion capture provides important information about functional alignment that is not provided by supine imaging or static motion capture. CLINICAL RELEVANCE Gait analysis may provide useful information to the surgeon during surgical planning of knee arthroplasties.

Use of robotic technology in cam femoroacetabular impingement corrective surgery

Cam type femoroacetabular impingement (FAI) is an anatomical disorder that can lead to osteoarthritis (OA) of the hip joint. With existing surgical options there is invariably a tendency for under‐resection or over‐resection of the cam lesion, both having dire consequences. This study assessed the application of robotic technology in the surgical management of cam FAI.