Tibial base design and patient morphology affecting tibial coverage and rotational alignment after total knee arthroplasty.

PURPOSE: To understand interactions between total knee arthroplasty tibial base design attributes, variations in tibial morphology, and the resulting tibial coverage and rotational alignment. METHODS: Tibial anthropometric measurements, including aspect ratio (medial-lateral width/anterior-posterior length) and tibial asymmetry, were taken for 14,791 total knee arthroplasty patients and compared with the ability of four different commercial tibial base designs to cover the resected plateau. The anthropometric measurements were also compared with the resulting tibial base rotation, which occurred when rotating the base to maximize coverage. RESULTS: All four tibial base designs resulted in similar coverage ranging from 80.2 (4.7)% to 83.8 (4.6)%. Mean tibial base rotation when placed to maximize coverage ranged from 3.7 (4.4)° (internal) to 3.8 (4.5)° (external) relative to the medial third of the tibial tubercle. More asymmetric tibiae and tibiae with a lower aspect ratios resulted in increased internal tibial base rotation. CONCLUSIONS: The four tibial base designs assessed provided similar levels of tibial bone coverage across the patient population, despite different design features. Rotating the tibial base to maximize coverage did not significantly increase the tibial coverage, but induced variability in tibial base alignment. Certain tibial anthropometrics may predispose particular patients to internal tibial base mal-rotation.

Influence of loading and activity on the primary stability of cementless tibial trays.

Several potential advantages exist for cementless tibial fixation including preservation of bone stock and increased longevity of fixation. However, clinical results have been variable, with reports of extensive radiolucent lines, rapid early migration, and aseptic loosening. The primary stability of an implant depends on the micromotion of the bone-implant interface, which depends on the kinematics and kinetics of the replaced joint. Finite element analysis was used to examine the micromotion for different activities (walking, stair ascent, stair descent, stand-to-sit, and deep knee bend) for three commercially available tibial tray designs. Similar trends were observed for all three designs across the range of activities. Stair ascent and descent generated the highest micromotions, closely followed by level gait. Across these activities, the mean peak (maximum) micromotions measured across the entire resected surface ranged from 64 to 78 (186-239) µm for PFC Sigma, 61-72 (199-251) µm for LCS Complete Duofix, and 92-106 (229-264) µm for LCS Complete. The peak micromotions did not necessarily occur at the peak loads. For instance, the peak micromotions for level walking occurred when there were low axial forces, but moderate varus-valgus moments. This highlights the need to examine the whole gait cycle to properly determine the initial stability of tibial tray designs. By exploring a range of activities and interrogating the entire resected surface, it is possible to differentiate between the relative performance of different implant designs.

Cross-linked glenoid prosthesis: a wear comparison to conventional glenoid prosthesis with wear particulate analysis.

Wear debris has been observed in shoulder arthroplasties that use an ultrahigh-molecular weight polyethylene (UHMWPE) glenoid component, and the biologic response to this debris contributes to aseptic loosening of the implant. The objective of this study was to assess the wear and particle morphology of a cross-linked UHMWPE prosthetic glenoid. To our knowledge, this is the first time a simulator with kinematic considerations for assessing wear has been used in a shoulder model. Shoulder wear testing was conducted on 2 groups of glenoids (n = 3 in each group) by use of an orthopaedic joint simulator to create worst-case scenario motions. One group was manufactured from conventional UHMWPE. The second was manufactured from 50-kGy cross-linked UHMWPE. The resulting wear rates for the conventional and cross-linked glenoid components were 46.7 +/- 2.6 mg/million cycles and 7.0 +/- 0.4 mg/million cycles, respectively. Particles isolated from the 2 groups showed similar morphologies; however, the calculated osteolytic potential of the cross-linked glenoid was significantly lower. The results of this study support the use of cross-linked UHMWPE glenoids in clinical applications.

Prosthetic positioning in total shoulder arthroplasty.

Accurate positioning of the prosthetic humeral head is necessary to reproduce normal glenohumeral kinematics and to avoid damage to the rotator cuff and impingement on the glenoid component or coracoacromial arch. Proper positioning of the head requires accurate placement of the stem and prosthetic designs that allow the head position to adapt to the variations in both normal and pathologic humeral anatomy. Glenoid malpositioning can lead to both humeral instability and increased stress of the glenoid component that may lead to premature glenoid loosening. This review summarizes the cadaveric and finite-element model that defines the abnormalities associated with humeral and glenoid component malpositioning.

Healing of reamed glenoid bone articulating with a metal humeral hemiarthroplasty: a canine model.

This study characterizes the healing response of the glenoid after spherical reaming and prosthetic humeral head replacement in a canine model of glenohumeral hemiarthroplasty. The right glenoid of twelve skeletally mature female dogs was reamed to a uniform radius of curvature, removing all cartilage down to bleeding subchondral bone. The glenoid was not resurfaced. The humeral head was replaced with a stemmed metal prosthesis. Post-surgery, the operated limbs were immobilized for seven days, with motion allowed ad libitum thereafter. Fluorescent bone labels were administered to identify bone formation. These procedures were not complicated by instability, infection or death. Six animals were euthanized at 10 week and six more at 24 week. The intact glenohumeral joints were evaluated by gross examination, assessment of glenoid concavity, and light microscopy of methylmethacrylate sections. At 10 week, vascular fibrous tissue partially covered the glenoid, maintaining a concave surface congruent with the prosthetic humeral head. New bone formed at the margin of the glenoid, and the density of the periarticular trabecular bone increased. At 24 week, the healing was more advanced; thick fibrocartilaginous tissue covered the entire glenoid surface. These results demonstrate that spherical glenoid reaming produced a consistent healing response characterized by remodelling of the reamed bony concavity to a congruent, living, smooth, securely attached interface articulating with the humeral prosthesis.