Total shoulder arthroplasty implants have evolved to include more anatomically shaped components that replicate the native state. The geometry of the humeral head is non-spherical, with the sagittal diameter of the base of the head being up to 6% (or 2.1-3.9 mm) larger than the frontal diameter. Despite this, many TSA humeral head implants are spherical, meaning that the diameter must be oversized to achieve complete coverage, resulting in articular overhang, or portions of the resection plane will remain uncovered. It is suspected that implant-bone load transfer between the backside of the humeral head and the cortex on the resection plane may yield better load-transfer characteristics if resection coverage was properly matched without overhang, thereby mitigating proximal stress shielding. Eight paired cadaveric humeri were prepared for reconstruction with a short stem total shoulder arthroplasty by an orthopaedic surgeon who selected and prepared the anatomic humeral resection plane using a cutting guide and a reciprocating sagittal saw. The humeral head was resected, and the resulting cortical boundary of the resection plane was digitized using a stylus and an optical tracking system with a submillimeter accuracy (Optotrak,NDI,Waterloo,ON). A plane was fit to the trace and the viewpoint was transformed to be perpendicular to the plane. To simulate optimal sizing of both circular and elliptical humeral heads, both circles and ellipses were fit to the filtered traces using the sum of least squares error method. Two extreme scenarios were also investigated: upsizing until 100% total coverage and downsizing until 0% overhang. Total resection plane coverage for the fitted ellipses was found to be 98.2±0.6% and fitted circles was 95.9±0.9%Cortical coverage was found to be 79.8 ±8.2% and 60.4±6.9% for ellipses and circles respectively. By switching to an ellipsoid humeral head, a small 2.3±0.3% (P < 0.001) increase in total coverage led to a 19.5±1.3%(P < 0.001) increase in cortical coverage. The overhang for fitted ellipses and circles was 1.7 ±0.7% and 3.8 ±0.8% respectively, defined as a percentage of the total enclosed area that exceeded the bounds of the humerus resections. Using circular heads results in 2.0 ±0.1% (P < 0.001) greater overhang.
The aims of this study were to determine the effect of osteophyte excision on deformity correction and soft tissue gap balance in varus knees undergoing computer-assisted total knee arthroplasty (TKA). A total of 492 consecutive, cemented, cruciate-substituting TKAs performed for varus osteoarthritis were studied. After exposure and excision of both cruciates and menisci, it was noted from operative records the corrective interventions performed in each case. Knees in which no releases after the initial exposure, those which had only osteophyte excision, and those in which further interventions were performed were identified. From recorded navigation data, coronal and sagittal limb alignment, knee flexion range, and medial and lateral gap distances in maximum knee extension and 90° knee flexion with maximal varus and valgus stresses, were established, initially after exposure and excision of both cruciate ligaments, and then also at trialling. Knees were defined as ‘aligned’ if the hip-knee-ankle axis was between 177° and 180°, (0° to 3° varus) and ‘balanced’ if medial and lateral gaps in extension and at 90° flexion were within 2 mm of each other.Aims
Methods
In posterior stabilised total knee replacement
(TKR) a larger femoral component is sometimes selected to manage the
increased flexion gap caused by resection of the posterior cruciate
ligament. However, concerns remain regarding the adverse effect
of the increased anteroposterior dimensions of the femoral component
on the patellofemoral (PF) joint. Meanwhile, the gender-specific
femoral component has a narrower and thinner anterior flange and
is expected to reduce the PF contact force. PF contact forces were
measured at 90°, 120°, 130° and 140° of flexion using the NexGen
Legacy Posterior Stabilized (LPS)-Flex Fixed Bearing Knee system
using Standard, Upsized and Gender femoral components during TKR.
Increasing the size of the femoral component significantly increased
mean PF forces at 120°, 130° and 140° of flexion (p = 0.005, p <
0.001 and p <
0.001, respectively). No difference was found in
contact force between the Gender and the Standard components. Among
the patients who had overhang of the Standard component, mean contact
forces with the Gender component were slightly lower than those
of the Standard component, but no statistical difference was found
at 90°, 120°, 130° or 140° of flexion (p = 0.689, 0.615, 0.253 and
0.248, respectively). Upsized femoral components would increase PF forces in deep knee
flexion. Gender-specific implants would not reduce PF forces.
