Validation of a model-based measurement of the minimum insert thickness of knee prostheses: a retrieval study.

INTRODUCTION: Wear of polyethylene inserts plays an important role in failure of total knee replacement and can be monitored in vivo by measuring the minimum joint space width in anteroposterior radiographs. The objective of this retrospective cross-sectional study was to compare the accuracy and precision of a new model-based method with the conventional method by analysing the difference between the minimum joint space width measurements and the actual thickness of retrieved polyethylene tibial inserts. METHOD: Before revision, the minimum joint space width values and their locations on the insert were measured in 15 fully weight-bearing radiographs. These measurements were compared with the actual minimum thickness values and locations of the retrieved tibial inserts after revision. RESULTS: The mean error in the model-based minimum joint space width measurement was significantly smaller than the conventional method for medial condyles (0.50 vs 0.94 mm, p < 0.01) and for lateral condyles (0.06 vs 0.34 mm, p = 0.02). The precision (standard deviation of the error) of the methods was similar (0.84 vs 0.79 mm medially and both 0.46 mm laterally). The distance between the true minimum joint space width locations and the locations from the model-based measurements was less than 10 mm in the medial direction in 12 cases and less in the lateral direction in 13 cases. CONCLUSION: The model-based minimum joint space width measurement method is more accurate than the conventional measurement with the same precision. Cite this article: Bone Joint Res 2014;3:289-96.

Bi-unicondylar knee prosthesis functional assessment utilizing force-control wear testing.

Recent in vivo studies have identified variations in knee prosthesis function depending on prosthesis geometry, kinematic conditions, and the absence/presence of soft-tissue constraints after knee replacement surgery. In particular, unicondylar knee replacements (UKR) are highly sensitive to such variations. However, rigorous descriptions of UKR function through experimental simulation studies, performed under physiological force-controlled conditions, are lacking. The current study evaluated the long-term functional performance of a widely used fixed-bearing unicompartmental knee replacement, mounted in a bi-unicondylar configuration (Bi-UKR), utilizing a force-controlled knee simulator during a simulated (ISO 14243) walking cycle. The wear behaviour, the femoral-tibial kinematics, and the incurred damage scars were analysed. The wear rates for the medial and the lateral compartments were 10.27 +/- 1.83 mg/million cycles and 4.49 +/- 0.53 mg/million cycles, respectively. Although constant-input force-controlled loading conditions were maintained throughout the simulation, femoral-tibial contact point kinematics decreased by 65 to 68 per cent for average anterior/posterior travel and by 58 to 74 per cent for average medial/lateral travel with increasing cycling time up to 2 million cycles. There were no significant differences in damage area or damage extent between the medial and the lateral compartments. Focal damage scars representing the working region of the femoral component on the articular surface extended over a range of 16-21 mm in the anterior-posterior direction. Kinematics on the shear plane showed slight variations with increasing cycling time, and the platform exhibited medial pivoting over the entire test. These measures provide valuable experimental insight into the effect of the prosthesis design on wear, kinematics, and working area. These functional assessments of Bi-UKR under force-controlled knee joint wear simulation show that accumulated changes in the UKR articular conformity manifested as altered kinematics both for anterior/posterior translations and internal/external rotations.

Effect of stair descent loading on ultra-high molecular weight polyethylene wear in a force-controlled knee simulator.

A loading protocol approximating forces, torques and motions at the knee during stair descent was developed from previously published data for input into a force-controlled knee simulator. A set of total knee replacements (TKRs) was subjected to standard walking cycles and stair descent cycles at a ratio of 70: 1 for 5 million cycles. Another set of implants with similar articular geometry and the same ultra-high molecular weight polyethylene (UHMWPE) resin (GUR 415), sterilization and packaging was tested with standard walking cycles only. Implant kinematics, gravimetric wear and surface roughness of the UHMWPE inserts were analysed for both sets of implants. Contact stresses were calculated for both loading protocols using a Hertzian line contact model. Significantly greater weight loss (p < 0.05) and more severe surface damage of UHMWPE inserts resulted with the walking + stair descent loading protocol compared to walking cycles only. Anterior-posterior (AP) tibiofemoral contact point displacements were lower during stair descent than walking, but not significantly different (p = 0.05). Contact stresses were significantly higher during stair descent than walking, owing to higher axial loads and the smaller radius of curvature of the femoral components at higher flexion angles. High contact stresses on UHMWPE components are likely to accelerate the fatigue of the material, resulting in more severe wear, similar to what is observed in retrieved implants. Thus the inclusion of loading protocols for activities of daily living in addition to walking is warranted for more realistic in vitro testing of TKRs.

Polyethylene damage and knee kinematics after total knee arthroplasty.

This study characterizes the relationship between in vivo knee kinematics and polyethylene damage by combining fluoroscopic analysis of tibiofemoral contact during dynamic activities and implant retrieval analysis in the same patients. Six patients (eight knees) underwent posterior cruciate ligament-retaining total knee arthroplasty. All patients participated in fluoroscopic analysis during a stair-rise and descent activity and treadmill gait an average of 18 months after arthroplasty, and articular contact was measured. Subsequently, all polyethylene tibial inserts were retrieved after an average of 26 months in vivo function: three at autopsy and five at revision. There was a statistically significant correlation between the damage location on the retrieved inserts and the articular contact location measured fluoroscopically during the activities. The femoral contact and polyethylene damage occurred predominantly on the posterior half of the tibial articular surface, and the damage pattern was largest in the compartment with the greatest range of in vivo femoral contact for each patient. This study showed that in vivo fluoroscopic analysis can predict the damage location on the polyethylene articular surface.

Locating femoral graft placement from lateral radiographs in anterior cruciate ligament reconstruction: a comparison of 3 methods of measuring radiographic images.

Graft positioning in anterior cruciate ligament (ACL) reconstruction is usually documented from lateral postoperative radiographs. The purpose of this study was to compare 3 measurement methods for femoral graft placement in 50 patients with ACL reconstruction. Intraoperative radiographic images were obtained and divided into 2 groups. The first group showed suboptimal projections, with out-of-plane rotations causing the femoral condyles to not be perfectly overlapped. The second group showed good projection, with optimal rotation and fully overlapped femoral condyles. In our study, only the measurement technique described by Amis produced data with the least measurement error when multiple observers assessed both groups. It is recommended that Amis' method be used to measure femoral ACL graft position so that reliable data are available for comparison between medical centers.

Patellar tendon graft position after anterior cruciate ligament reconstruction. Interobserver variability on lateral radiographs.

We compared the reliability and validity of graft position measurements made by 4 orthopedic surgeons on intraoperative radiographs obtained using fluoroscopic control and postoperative radiographs obtained from the same 17 patients 6 weeks after ACL reconstruction. Measurements from postoperative radiographs varied significantly more than those from intraoperative radiographs. There was little agreement between the postoperative and intraoperative measurements of the tibial and femoral graft position. We conclude that postoperative radiographs are not a sufficient tool for assessing graft placement after ACL reconstruction using patellar tendon autografts. In order to consider graft position in follow-up studies and to compare results from various surgeons, we suggest intraoperative fluoroscopy to produce radiographs for accurate and reliable measurements.

Wear patterns on tibial plateaus from varus and valgus osteoarthritic knees.

The size and location of articular cartilage wear was assessed on 106 varus and 37 valgus osteoarthritic tibial plateaus resected during total knee arthroplasty. Anterior cruciate ligament integrity was assessed intraoperatively, and calibrated digital images were used to measure the wear patterns. Complete anterior cruciate ligament deficiency was seen in 25% of the varus and 24% of the valgus knees. Wear patterns on anterior cruciate ligament intact and attenuated varus tibial plateaus occurred in the middle to anterior aspect of the medial plateau. Anterior cruciate ligament deficient varus plateaus had significantly larger wear areas located more posterior on the medial plateau. In contrast, anterior cruciate ligament intact and deficient valgus tibial plateaus had wear located posterior to the center of the lateral plateau. Anterior cruciate ligament integrity is a discrete feature of advanced osteoarthritis that strongly influences the articular wear patterns. The anterior cruciate ligament deficient wear patterns show a wear mechanism that is consistent with the posterior femoral subluxation and posterior tibiofemoral contact observed after acute anterior cruciate ligament rupture. These observations provide insight into the altered knee mechanics that exist in osteoarthritic knees and the resulting mechanical factors that contribute to degenerative changes.

Wear analysis of a retrieved hip implant with titanium nitride coating.

There is increasing interest in using surface modification technology to improve the wear properties of titanium alloy and limit articular surface wear of metal and polyethylene components. This report details the in vivo wear performance of titanium nitride coating on a retrieved hip implant obtained postmortem from a low demand patient 1 year after total hip arthroplasty. Analysis of the well-functioning implant revealed that wear debris can originate from a titanium nitride coated femoral head, as delaminated surface asperities, and manifest as adhesive wear on the articular surface. The wear observed on this implant indicates that rigorous testing and evaluation of titanium nitride coating technology should be conducted prior to widespread use on total joint implants.

Initial stability of uncemented hip stems: an in-vitro protocol to measure torsional interface motion.

The difficulty in quantitatively assessing the inherent variables of surgical stem insertion and interfemur differences continues to be a problem in experimental methodologies which assess hip stem stability. An in-vitro torsional stability protocol was developed which limited the mechanical testing variability and provided a reproducible micromotion measurement of an uncemented stem in synthetic composite femurs. Using a controlled mechanical stem insertion resulted in less interfemur variability within each group with the coefficient of variation being reduced from 35% overall to less than 20%. Femurs with shallow stem insertion depths had significantly larger micromotion than femurs having deep stem insertion depths. The sensitivity of the experimental protocol and the synthetic composite femurs to the varied functional behaviour of three different stem designs was demonstrated. The stem with a hollowed anterior-to-posterior proximal section experienced significantly more motion than the two stems with full proximal sections, reinforcing the need for proximal contact to ensure minimal micromotion in torsional loading.