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The tibial component in total knee arthroplasty (TKA) is often chosen to maximize coverage of the tibial cut, which can result in excessive internal rotation of the component. Optimal rotational alignment may require a smaller baseplate with suboptimal coverage that could threaten fixation. We asked: "does undersizing the tibial component of a cementless TKA to gain external rotation increase the risk of bone failure?"We developed computational finite element (FE) analysis models from the computed tomography (CT) scans of 12 patients scheduled for primary TKA. The models were implanted with a cementless tibial baseplate that maximized coverage and one or two sizes smaller and externally rotated by 5°. We calculated the risk of bone collapse under loads representative of stair ascent.Undersizing the implant increased the area at risk of collapse for eight patients. However, the area at risk of collapse for the undersized implant (range, 5.2%-16.4%) was no different (P = .24) to the optimally sized implant (range, 4.5%-17.9%). The bone at risk of collapse was concentrated along the posterior edge of the implant. The area at risk of collapse was not proportional to implant size, and for four subjects undersizing the implant actually decreased the area at risk of collapse.While implants should maximize coverage of the tibial cut and seek support on dense bone, undersizing the tibial component to gain external rotation had minimal impact on the load transfer to the underlying bone. This FE analysis model of a cementless tibial baseplate may require further validation and additional studies to investigate the long-term biomechanical effects of undersizing the tibial baseplate. In conclusion, while surgeons should strive to use the appropriate tibial baseplate for each patient, our model identified only minor biomechanical consequences of undersizing the implant for the immediate postoperative bone-implant interaction and implant subsidence.

Background The bone morphologies of intact knees were measured and compared between Chinese and Caucasian populations. However, to assess if distinct designs of implants are necessary for the Chinese population owing to different morphologies and sizes, the knee measurements after osteotomy performed in total knee arthroplasty were evaluated. Methods Thirty-seven Caucasian and 50 Chinese patients' knees were examined using computed tomography scans. Mimics were applied to reconstruct 3-dimensional bone models. Dimensions of the 3-dimensional knee models and simulated bone resections during total knee arthroplasty were measured using Geomagic Studio and Pro/ENGINEER. The morphologic measurements of the native and resected femur and tibia included the anteroposterior (AP) depth, mediolateral (ML) width, notch width, knee physical valgus angle, tibial slope angle, and the ML-to-AP ratio of the femur, tibia, and resected femur. Statistical analysis was performed using the independent samples t test and the Pearson correlation coefficient in SPSS for Windows. Values of P < .05 were considered significant. Results No measurements were significantly different between the Chinese and Caucasian knees. However, the Chinese female showed significant differences compared with the Chinese male on distal femoral measurements both presimulated and postsimulated osteotomy such as a smaller mean ML-to-AP ratio in presimulated (1.3 ± 0.1) and postsimulated (1.3 ± 0.1) osteotomy. Conclusion The necessity of designing a full set of total knee components specifically for the Chinese population is still undetermined. However, we suggest designing femoral components specific for the Chinese females because of different postosteotomy distal femoral ML-to-AP ratio between the Chinese males and the Chinese females.

Newer arthroplasty designs claim to provide superior range of motion (ROM) and greater stability than their predecessors. However, there is no way to compare ROM of implant systems in an equivalent anatomical environment in a clinical setting. This study used computer-aided design to compare ROM after hip resurfacing, 28 mm THA, 36 mm THA, and anatomic dual mobility (ADM) THA in 3D models of 5 cadaver pelvises. ROM to impingement was then tested in 10 different motions and a one-way ANOVA was used to compare results. The hip resurfacing resulted in restricted ROM compared to the other 3 models in all motions except adduction. The ADM, 36 mm, and 28 mm THA resulted in similar ROM. Dual mobility constructs provide comparable ROM in patients where large head THA is not appropriate.

Unicondylar knee arthroplasty (UKA) is considered an alternative to total knee arthroplasty for patients who have arthritis limited to one compartment of the knee. This study examined surface damage of 3 contemporary UKA designs that were retrieved at revision surgery. Two of the UKA designs were fixed bearing and one was mobile bearing. Demographic information was collected, as well as information about the implants used at revision surgery. Articular surface damage was greater in the fixed-bearing designs as compared to the mobile bearing, although the mobile-bearing implants had significantly shorter length of implantation. Backside damage was also graded for the mobile bearing and when combined with articular wear resulted in overall damage scores higher than both fixed-bearing designs. The fixed-bearing designs showed delamination and surface deformation, whereas the mobile bearing had no evidence of these damage modes. However, mobile-bearing components showed other types of wear, and significant wear damage was present on the bearing surfaces of the mobile-bearing implants despite a short time of implantation. At the time of conversion to a total knee arthroplasty, more than 50% of cases required the use of stems, augments, or constrained inserts for the tibial reconstruction. In conclusion, wear modes differed among UKA prosthesis designs. Revision of a UKA to a total knee arthroplasty remains complex with the tibial preparation more complicated than in the primary setting.

The effect of fixed spinal deformities on a functional pelvis from standing to sitting is not fully understood. We aimed to assess the change in preoperative sagittal pelvic tilt angle (SPTA) from standing to sitting in patients undergoing total hip arthroplasty, comparing flexible and fixed spinal deformities.Between July 2011 and October 2011, 68 consecutive unilateral total hip arthroplasties were implanted in 68 patients with a mean age of 71 ± 6 years. Fixed spinal deformity was defined as10° of the change in SPTA from standing to sitting. Preoperative radiographic evaluation included standing (weight-bearing) anteroposterior and lateral pelvic and lumbosacral radiographs and a sitting lateral pelvic radiograph.The mean standing and sitting SPTA was 3.7° of anterior tilt and 17.7° of posterior tilt, respectively (change of 21.4 ± 12.5°). Seventy-five percent had flexible pelvises, all of which had a posterior tilt from standing to sitting. One patient in the fixed pelvis (1.4%) had a loss of posterior tilt from standing to sitting. The mean change of SPTA from standing to sitting in the fixed and flexible pelvis groups was 5.9 ± 3.5° to 26.7 ± 9.6° of posterior tilt, which was statistically significant (P.05).There was a significant change in sagittal pelvic tilt from standing to sitting, especially in patients with a flexible spine, in which the functional anteversion increases with sitting. The patients with a fixed pelvis had significantly less SPTA in standing (less anteversion) with less posterior sagittal tilt in sitting, which should be incorporated in cup positioning.

We hypothesized that the reduction in toughness that accompanies elevated levels of cross-linking in ultrahigh-molecular weight polyethylene would lead to increased wear and damage in acetabular cups under conditions of impingement with the neck of the femoral component. Test cups were exposed to 2.8, 10, and 20 Mrad of gamma irradiation and were tested in a simulator, incorporating impingement during every wear cycle. Wear behavior was compared based on wear rate, observations of damage, and the number and morphology of wear particles. The 10-Mrad cups had the lowest rate (15.2 +/- 5.1 mg/Mc), followed by the 2.8-Mrad (40.1 +/- 4.2 mg/Mc) and 20-Mrad (67.9 +/- 24.2 mg/Mc) groups. The wear damage was most severe in the 20-Mrad cups, although all 3 groups showed pitting and delaminating. Wear rates and damage of highly cross-linked polyethylene cups were significantly higher in an impingement wear test than in previous simulator studies.

Although new generation alumina ceramics have exhibited a reduced incidence of fracture, concern still persists about the behavior of ceramic acetabular liners under impact conditions. The objective of this study was to explore whether fracture of a new generation alumina ceramic liner was likely to occur in vivo. Ceramic liners were impacted with forces of 23, 21, 15, and 12 kN (n = 3 at each force). At 23 kN, all 3 ceramic liners fractured on the first impact; at 12 kN none of the ceramic liners fractured after 20 impacts. The threshold force of 12 kN is large in comparison with estimated physiologic forces on the hip during falls or stumbling, suggesting that ceramic liner fracture is not a definite consequence of liner impact. Deposited by bulk import 27/09/13 RB

s e9 Paper #9 Unicondylar Knee Implant Retrieval Theodore T. Manson, MD, Geoffrey H. Westrich, MD, Natalie Kelly, BS, Joseph Lipman, MS, Timothy M. Wright, PhD Introduction: Wear behavior of unicondylar knee arthroplasty (UKA) should be influenced by implant design. We compared wear patterns among 3 modern designs: Oxford mobile bearing (Biomet), Repicci fixed bearing with an all polyethylene tibia (Biomet), and Miller-Galante fixed bearing with a metal-backed tibia (Zimmer). Methods: Between 1999 and 2008, 44 tibial components were retrieved as part of an ongoing retrieval program. Wear damage was graded on a subjective 0 to 3 scale for 7 modes: scratching, pitting, burnishing, embedded debris, abrasion, and delamination. A cumulative score was assigned each component, in addition to scores for each mode. Results: Average patient age at implant removal was 61.7 years with no differences among the groups. Cumulative wear scores were higher for Repicci and Miller-Galante prostheses (34.8 and 33.6, respectively) than for Oxford prostheses (22.58; P b .01). However, the backside of the Oxford prostheses also showed wear damage with an average score of 16.3. Repicci and Miller-Galante had higher delamination scores (.89 and .75, respectively), while no Oxford prosthesis showed delamination. Likewise, Repicci and Miller-Galante had higher surface deformation scores (1.81 and 1.63) while the Oxford showed no surface deformation. Embedded debris was more common in the Repicci (0.6) than either the Oxford (0.19) or Miller-Galante (0.2) prostheses (P b .01). Conclusion: The Oxford prostheses showed less influence of shear-induced wear patterns (surface deformation and delamination) than the Repicci and Miller-Galante prostheses. Oxford prostheses revealed less overall wear on the bearing surfaces, though the presence of a mobile bearing provided another source for wear damage. The absence of delamination and surface deformation in the Oxford implants is consistent with the mobile bearing, providing less constraint to the bearing surfaces. The main types of wear present in our retrieval specimens could be minimized with a change to highly cross-linked polyethylene.

Introduction: Concern for stability and increased range of motion in THA has increased popularity of large femoral heads, while thin polyethylene liners have allowed acetabular component thickness to remain constant. Highly cross-linked polyethylene improves wear, but is accompanied by reduced toughness. Decreasing thickness may increase wear or cause gross fracture, especially at extremes of motion when contact between head and acetabulum nears the liner edge. We determined the effect of liner thickness on wear with large diameter heads in a near impingement model.