Constrained cups appear incapable of meeting the demands of revision THA.

BACKGROUND: Failure rates of constrained cups for treating recurrent dislocation in revision THA range from 40% to 100%. Although constrained liners are intended to stabilize the hip by mechanically preventing dislocation, the resulting loss of range of motion may lead to impingement and, ultimately, implant failure. QUESTIONS/PURPOSES: We therefore documented the mechanisms of failure of constrained acetabular cups in revision THA and determined the type and severity of damage (wear, fracture, and impingement) that occurs in situ. METHODS: We retrieved 57 constrained components of four different designs at revision THA and examined for the presence of rim impingement, oxidation, cracks within the liner, backside wear, pitting, scratching, abrasion, burnishing, and the presence of embedded particles. Articular wear was calculated from the volume of the concave articular bearing surface, which was measured using the fluid displacement method. RESULTS: Failure of the locking ring was responsible for 51% of failures, whereas 28% of revisions were the result of acetabular cup loosening, 6% backside wear, and 22% infection. Impingement damage of the rim of the polyethylene liner was seen in all retrievals with moderate or severe damage in 54%. The average volumetric wear rate of the articular surface was 95 mm(3)/year. CONCLUSIONS: Failure of the locking liner ring and loosening of the acetabular cup are the primary causes of mechanical failure with constrained liners; polyethylene is an inadequate material for restricting motion of the hip to prevent instability. The durability of these devices is unlikely to improve unless the mechanical demands are modified through increased range of motion leading to less frequent rim impingement.

Biomechanical and clinical perspectives on nighttime bracing for adolescent idiopathic scoliosis.

The present review article aims at providing an update on the basic science and clinical information underlying the use of nocturnal braces for adolescent idiopathic scoliosis. The use of nocturnal braces has been dictated by the encouraging results recorded by some studies on part-time bracing, combined with increasing concerns on poor patient compliance noted with the use of full-time bracing. The cardinal feature of nighttime braces lays in their ability to hypercorrect the scoliotic curvature, thereby eliminating the asymmetric water accumulation that occurs in the apical and adjacent intervertebral discs, thus restoring a close-to-normal force application through the Hueter-Volkmann principle and preventing curve progression. The two nighttime braces mostly used hypercorrect the spine through different mechanisms, one acting by bending the spine and the other by the application of opposing forces. Based on the clinical results available, nighttime braces constitute an attractive option for single-major lumbar/thoracolumbar curves not exceeding 35 degrees in magnitude. Multi-center, randomized studies using strict criteria set forth by the Scoliosis Research Society and the Study group On Scoliosis Orthopaedic and Rehabilitation Treatment are needed to better define the role of nocturnal bracing in the conservative treatment of adolescent idiopathic scoliosis.

Hip impingement: beyond femoroacetabular.

In the last 20 years, femoroacetabular impingement has been at the forefront of clinical practice as a cause of hip pain in young adults. As arthroscopic techniques for the hip continue to evolve, the possible presence of a new group of conditions creating mechanical conflict in and around the hip joint (ischiofemoral, subspine and iliopsoas impingement) has recently been elucidated whilst interest in already known 'impingement' syndromes (pelvic-trochanteric and pectineofoveal impingement) is now revived. This article attempts to increase awareness of these relatively uncommon clinical entities by describing their pathomorphology, contact mechanics, treatment and published results available to present. It is hoped that such knowledge will diversify therapeutic options for the clinician, thereby improving outcomes in a small but not negligible portion of patients with previously unexplained persistent symptoms.

The position of the centre of the femoral head relative to the midline of the pelvis: a consistent landmark in total knee replacement surgery.

BACKGROUND: Femoro-tibial malalignment in excess of 3° is a recognised contributor of early mechanical failure in total knee replacement (TKR). Knowledge of the location of the centre of the femoral head is a pre-requisite to identification of the mechanical axis of the femur and can facilitate optimal component orientation. We investigated variation in the location of the centre of the femoral head relative to the midline of the pelvis. METHODS: We analysed the pelvic radiographs of 150 patients with unilateral total hip replacements. The perpendicular distance from the centre of the femoral head of the non-operated hip to the centre of pubic symphysis was measured. RESULTS: The mean distance from the centre of the femoral head to the pubic symphysis was 89.2mm (standard deviation, 5.7 mm). Patient height strongly correlated with this distance (r=0.53, p<0.01), as did the diameter of the femoral head (r=0.59, p<0.01). The latter was significantly larger in men than in women (50.9 mm vs. 44.5mm, p<0.01). CONCLUSION: The results demonstrate that the position of the centre of the femoral head has very little variability, irrespective of patient age or body-mass index. If the gender-specific mean femoral head to midline distance is used to estimate the location of the femoral head centre, a line from this point to the centre of the femoral condyles will deviate from the true mechanical axis by no more than 1.5°, in 98% of cases.

Brace technology thematic series: the dynamic derotation brace.

BACKGROUND: The dynamic derotation brace (DDB) was designed in Greece in 1982, as a modification of the Boston brace. It is a custom-made, underarm spinal orthosis featuring aluminium blades set to produce derotating and anti-rotating effects on the thorax and trunk of patients with scoliosis. It is indicated for the non-operative correction of most curves, barring the very high thoracic ones, (when the apex vertebra is T5 or above). The purpose of this article is to familiarize physicians with the DDB, analyze the rationale behind its design, and present the published results of its application. DESCRIPTION & PRINCIPLES: The key feature of the DDB is the addition of the aluminium-made derotating blades posteriorly. These function as a force couple, which is added to the side forces exerted by the brace itself. Corrective forces are also directed through pads. One or more of previously proposed pathomechanical models of scoliosis may underline the corrective function of the DDB: it may act directly on the apical intervertebral disc, effecting correction through the Heuter-Volkman principle; the blades may produce an anti-rotatory element against the deforming "spiral composite muscle trunk rotator"; or it may alter the neuro-motor response by constantly providing new somatosensory input to the patient. RESULTS: Based on measurements of the Cobb and Perdriolle angles, up to 82% of patients remained stable or improved with the use of the DDB. Results have varied, though, depending on the type/location of the deformity. The overall results showed that 35% of the curves improved, 46% remained stable and 18% became worse, as assessed by measuring the Cobb angle. The DDB has also been shown to improve cosmesis (except for right thoracic curves) and leave several aspects of patient quality of life unaffected during use. CONCLUSION: Conservative treatment of idiopathic scoliosis using the DDB has shown favorable results. Thoracic curves appear more resistant to both angular and rotatory correction. The published outcome data on the DDB support our belief that the incorporation of aluminium blades to other orthoses would likely improve their efficacy.

Night-time braces for treatment of adolescent idiopathic scoliosis.

PURPOSE: This review article aims at providing an update on the basic science and clinical information underlying the use of nocturnal braces for adolescent idiopathic scoliosis. METHOD: The National Institutes of Health online medical database (Medline) was used to retrieve all articles pertinent to clinical research on night-time bracing; data were also collected from manufacturing companies. Additional information was obtained from literature on pathomechanics of scoliosis. RESULTS: The cardinal feature of night-time braces lays in their ability to hypercorrect the scoliotic curvature, thereby eliminating the asymmetric water accumulation that occurs in the apical and adjacent intervertebral discs. Previously wedge-shaped discs resume a nearly cylindrical configuration, thus restoring a close-to-normal force application to the endplates through the Hueter-Volkmann principle and preventing curve progression. The two night-time braces mostly used hypercorrect the spine through different mechanisms. CONCLUSIONS: On the basis of clinical results available, night-time braces constitute an attractive option for single-major lumbar/thoracolumbar curves not exceeding 35 degrees in magnitude. Multi-center, randomized studies using strict criteria set forth by the Scoliosis Research Society (SRS) and the Society on Spinal Orthopaedic and Rehabilitation Treatment (SOSORT) are needed to better define the role of nocturnal bracing in the conservative treatment of adolescent idiopathic scoliosis.