Rotational constraint in posterior-stabilized total knee prostheses.

Rotational stresses from box-post impingement have been implicated in the loosening of posterior-stabilized total knee prostheses. A bench model was constructed to assess the forces generated by tibiofemoral rotation. Rotational torque under load was measured in two different posteriorstabilized total knee prostheses using an axial-torsion load cell at 0 degrees, 20 degrees, and 40 degrees flexion over 20 degrees internal and external rotation. The Sigma posterior-stabilized prosthesis generated little torque through 5 degrees internal and external rotation. An increase in torque then occurred because of box-post impingement, generating peak torques of 17 to 18 N-m at 12 degrees to 14 degrees rotation. The bench model produced the same deformation of the polyethylene post as seen on retrieved specimens. The Scorpio posterior-stabilized prosthesis had a relatively continuous rise in generated torque from tibiofemoral conformity. Box-post impingement did not occur resulting in 32% lower torque between 12 degrees and 14 degrees rotation. Peak rotational torques of 15 to 16 N-m were reached at 19 degrees to 20 degrees rotation. Tibiofemoral conformity is the primary source of rotational constraint. Box-post impingement can be a source of additional rotational constraint. Depending on specific design features, small changes in relative tibiofemoral component rotation can more than double the generated torque. Axial rotation of the knee in vivo can generate substantial torque. Relative tibiofemoral rotational position is an important factor influencing component function and fixation.

Effects of titanium prosthesis, offset and size of field of view on bone mineral density measurements using quantitative computed tomography.

To estimate the accuracy of quantitative computed tomography (QCT) as a method to measure bone mineral density (BMD) in the vicinity of a titanium prosthesis, we investigated the effects of (1) titanium prosthesis, (2) offset of the longitudinal axis of the bone to be examined from that of the gantry of the CT scanner, (3) size of the field of view (FOV) and (4) the combination of these effects on CT based measurements of mineral density of cortical and cancellous bone specimens. 14 bovine cortical bone parallelepipeds and 14 bovine cancellous bone parallelepipeds were used in this investigation. The bone specimens were scanned with and without a titanium prosthesis, when centered in the gantry of the CT scanner and offset from the axis of the gantry of the CT scanner at a distance of 14 cm. Image data were then reconstructed separately with a FOV of 10 cm and 30 cm. All BMD values taken from CT images obtained under different scanning condition were compared with the BMD values of the corresponding bone parallelepiped obtained under standard condition (centered in the gantry of the CT scanner, 10 cm FOV, without titanium prosthesis). When centered in the gantry of the CT scanner, the mean relative difference of BMD measurements caused by the presence of the titanium prosthesis was less than 1% for both cortical bone and cancellous bone. Size of the FOV had a negligible effect on BMD measurements. Offset at 14 cm, however, caused a significant difference in BMD measurements (p < 0.001). It was concluded that titanium prosthesis did not interfere with BMD measurements of cortical and cancellous bone when both the specimen and prosthesis were centered in the gantry of the CT scanner. However, the effect on BMD measurements of offset at 14 cm combined with the presence of a titanium prosthesis in bone was significant.

Cerclage wiring technique after proximal femoral fracture in total hip arthroplasty.

Cerclage wires have been used to stabilize proximal femoral cracks after stem insertion in cementless total hip arthroplasty. The objective of this study was to determine the optimal number and orientation of cerclage wires necessary to prevent stem subsidence and crack propagation. The crack was stabilized by 1, 2, or 3 wires placed either normal to the femoral neck axis or normal to the crack. The femora were compressed to 2,670 N while measuring crack opening and stem subsidence. Wires placed normal to the crack allowed less stem subsidence by 3.17 mm and less crack opening by 1.55 mm compared with wires placed normal to the neck. The addition of multiple wires reduced subsidence by 50% and reduced crack opening to <1 mm. Medial and anterior calcar cracks are best stabilized by at least 2 cerclage wires that are placed normal to the crack.

Ultrasound velocity and broadband attenuation as predictors of load-bearing capacities of human calcanei.

The main purpose of this study was to determine whether calcaneal ultrasound parameters, measured in the mediolateral direction, reflect load-bearing capacities of human calcanei. Broadband ultrasound attenuation (BUA) and ultrasound velocity (UV) were measured in 20 cadaveric calcanei with a mean age of 74.1 (SD 8.8). Normalized BUA (nBUA) was determined by dividing BUA by the calcaneal thickness obtained using a pulse-echo technique. The bone mineral density (BMD) of each calcaneus was measured by quantitative computed tomography. The calcanei were embedded in PMMA to simulate the midstance physiologic orientation during compressive testing in the load-bearing direction. The failure load, stiffness, and energy absorption were determined for each calcaneus. It was shown that BMD was well correlated with all ultrasound parameters (P < 0.0001). BMD, BUA, nBUA, and UV were all significantly associated with calcaneal failure load, stiffness, and energy absorption capacity (P < 0.05). nBUA was found to be the strongest predictor of all compressive properties. BUA and BMD demonstrated similar predictability of stiffness and energy absorption capacity, however, BUA showed a more significant relationship to the failure load of the calcaneus than did BMD. UV was found to be inferior to BMD, as well as BUA or nBUA, in assessing failure load, stiffness, and energy absorption capacity. It was also shown that nBUA was superior to BUA in the assessment of load-bearing capacity, but not in the prediction of BMD. Multivariate regression analysis showed that the combination of BUA or nBUA with UV did not improve the predictability of failure load, stiffness, and energy absorption capacity over that of BUA or nBUA alone (P > 0.5).