Effect of material selection on tibial post stresses in posterior-stabilized knee prosthesis.

AIMS: The material and design of knee components can have a considerable effect on the contact characteristics of the tibial post. This study aimed to analyze the stress distribution on the tibial post when using different grades of polyethylene for the tibial inserts. In addition, the contact properties of fixed-bearing and mobile-bearing inserts were evaluated. METHODS: Three different grades of polyethylene were compared in this study; conventional ultra high molecular weight polyethylene (UHMWPE), highly cross-linked polyethylene (HXLPE), and vitamin E-stabilized polyethylene (VEPE). In addition, tibial baseplates with a fixed-bearing and a mobile-bearing insert were evaluated to understand differences in the contact properties. The inserts were implanted in neutral alignment and with a 10° internal malrotation. The contact stress, von Mises stress, and equivalent plastic strain (PEEQ) on the tibial posts were extracted for comparison. RESULTS: The stress and strain on the tibial post for the three polyethylenes greatly increased when the insert was placed in malrotation, showing a 38% to 56% increase in von Mises stress and a 335% to 434% increase in PEEQ. The VEPE insert had the lowest PEEQ among the three materials. The mobile-bearing design exhibited a lower increase in stress and strain around the tibial posts than the fixed-bearing design. CONCLUSION: Using VEPE for the tibial component potentially eliminates the risk of material permanent deformation. The mobile-bearing insert can help to avoid a dramatic increase in plastic strain around the tibial post in cases of malrotation. The mobility allows the pressure to be distributed on the tibial post and demonstrated lower stresses with all three polyethylenes simulated. Cite this article: Bone Joint Res 2020;9(11):768-777.

Polyethylene-Based Knee Spacer for Infection Control: Design Concept and Pre-Clinical In Vitro Validations.

Antibiotic-loaded polymethyl methacrylate (PMMA) has been widely applied in the treatment of knee periprosthetic joint infections. However, problems with antibiotic-loaded PMMA-based spacers, such as structural fracture and implant dislocation, remain unresolved. A novel polyethylene-based spacer, designed with an ultra-congruent articulating surface and multiple fenestrations, was introduced in the current study. Validation tests for biomechanical safety, wear performance, and efficacy of antibiotic cement were reported. During cycle fatigue testing, no tibial spacer failures were observed, and less wear debris generation was reported compared to commercial PMMA-based spacers. The volumetric wear of the novel spacer was within the safety threshold for osteolysis-free volumetric wear. An effective infection control was demonstrated despite the application of lesser antibiotic cement in the 30-day antibiotic elution test. The tube dilution test confirmed adequate inhibitory capabilities against pathogens with the loaded antibiotic option utilized in the current study. The novel polyethylene-based knee spacer may offer sufficient biomechanical safety and serve as an adequate carrier of antibiotic-loaded cement for infection control. Further clinical trials shall be conducted for more comprehensive validation of the novel spacer for practical application.

Electromechanical delay of the vastus medialis obliquus and vastus lateralis in individuals with patellofemoral pain syndrome.

STUDY DESIGN: Case-control study. OBJECTIVE: To examine electromechanical delay (EMD) of the vastus medialis obliquus (VMO) and the vastus lateralis (VL) in individuals with patellofemoral pain syndrome (PFPS). BACKGROUND: EMD is a mechanical property of muscles related to protective reflex and sports performance. The time duration of the EMD can be shortened with strength training and, conversely, can be lengthened secondary to immobilization. However, it is unclear if EMD between various components of the quadriceps is affected in individuals with PFPS. METHODS: Twenty-six individuals with PFPS and 26 healthy volunteers were studied. The VMO and VL were electrically stimulated to evoke muscle twitches. Ultrasound was used to assess patellar movement elicited by the muscle twitch. The time from the onset of electrical stimulation to the onset of patellar movement was measured as the EMD. The EMDs of the VMO and VL were compared between groups using a mixed-model analysis of variance. RESULTS: Subsequent to a significant interaction (P<.001), post hoc analysis indicated that the EMD of the VMO was longer (PFPS, 37.3 ± 0.7 milliseconds; control, 25.9 ± 0.7 milliseconds; P<.001) and the EMD of the VL was shorter (PFPS, 18.4 ± 0.5 milliseconds; control, 25.1 ± 0.5 milliseconds; P<.001) in the PFPS group. Therefore, in the individuals with PFPS, the EMD of the VMO was significantly longer than that of the VL (P<.001), which was not the case for those in the control group (P = .20). CONCLUSION: The mechanical properties of the VMO and VL may be altered in patients with PFPS.J Orthop Sports Phys Ther 2012;42(9):791-796, Epub 2 August 2012. doi:10.2519/jospt.2012.3973.

Arthroscopic treatment of rotator cuff tears with shoulder stiffness: a comparison of functional outcomes with and without capsular release.

BACKGROUND: In a single arthroscopic surgery for rotator cuff tears with a stiff shoulder, closed manipulation with or without arthroscopic capsular release has been combined with the repair. Few studies have reported functional results after this surgery, and a clinical comparison has not been reported. PURPOSE: To compare functional outcomes with and without capsular release in arthroscopic treatment of rotator cuff tears with a stiff shoulder. STUDY DESIGN: Cohort study; Level of evidence, 3. METHODS: Among patients who underwent arthroscopic rotator cuff repairs with a concomitant stiff shoulder, 34 patients were treated with either manipulation and arthroscopic capsular release (group A; n = 18) or manipulation without capsular release (group B; n = 16). Preoperative mean passive forward flexion was 118° in group A and 117° in group B, whereas external rotation at the side was 28° and 29°, respectively. All patients were evaluated at a minimum 2-year follow-up in terms of visual analog scale for pain, muscle power, range of motion, Constant score, modified American Shoulder and Elbow Surgeons (ASES) shoulder evaluation form, and modified University of California at Los Angeles (UCLA) scores. RESULTS: The mean modified ASES score was 90.3 for group A and 87.7 for group B (P = .086). The mean UCLA scores were 34.3 and 33.0, respectively (P = .247). The 2 groups showed a significant difference in forward flexion and external rotation post operatively, as group B recovered more slowly. There was no difference in the mean visual analog scale for pain at the last follow-up, but a 13° difference in forward flexion (P < .001) and a 10° difference in external rotation (P =.001) were seen. CONCLUSION: Overall satisfactory results could be achieved by either method, whereas rapid recovery and improvement of range of motion could be achieved by using a single arthroscopic repair and concomitant release for patients with rotator cuff tears with stiff shoulder.

Non-hardware posterior cruciate ligament reconstruction using knot/press-fit technique with periosteum-enveloped hamstrings tendon autograft.

BACKGROUND: Several posterior cruciate ligament reconstruction procedures have been proposed to manage ruptures. PURPOSE: This study was intended to present the clinical results of non-hardware reconstruction of posterior cruciate ligament using a knot/press-fit technique with periosteum-enveloped autogenous hamstring tendons. STUDY DESIGN: Case series; Level of evidence, 4. METHODS: This arthroscopically assisted technique was used in 33 patients with posterior cruciate ligament ruptures. In this non-hardware technique, semitendinosus and gracilis tendons were prepared as 2 loops with knots. Each loop was enveloped in periosteum. After passing a bottleneck femoral tunnel, the grafts were fixed with a press-fit method (knotted grafts lodging in the bottleneck of the femoral tunnel). Simultaneously, the intra-articular opening of the tibial tunnel was filled with the periosteum-enveloped portion of the graft. A tie with Mersilene tape over a bone bridge for each tendon loop was used for tibial fixation. RESULTS: The average follow-up was 35 months (range, 24-60 months). Clinical assessments included Tegner score, Lysholm knee score, International Knee Documentation Committee scores, thigh muscle assessment, and radiographic evaluation. The stress radiography results for posterior displacement changed from 13.7 ± 2.1 mm preoperatively to 4.8 ± 1.1 mm postoperatively. Average preinjury Tegner score was 5.9 (range, 3-9), decreasing to 2.9 (range, 2-5) preoperatively and increasing to 5.2 (range, 2-9) postoperatively. The Lysholm score increased from 58.5 ± 5.2 to 94.2 ± 4.1 (P < .01). Finally, 26 patients (89.7%) were assessed as nearly normal by International Knee Documentation Committee guidelines. CONCLUSION: The study revealed satisfactory clinical subjective and objective results at a minimum of 2 years' follow-up. Without using hardware, this alternative technique has the advantage of no need for removal of hardware and potentially easier magnetic resonance imaging interpretation and revision surgery.

Correlation between proprioception, muscle strength, knee laxity, and dynamic standing balance in patients with chronic anterior cruciate ligament deficiency.

Proprioception and muscle strength are both reported to influence single-limb stance balance in patients with chronic anterior cruciate ligament (ACL) injuries. However, the effects of these parameters on dynamic stance balance in such patients are currently unknown. This study was undertaken to ascertain whether proprioception, muscle strength, and knee laxity are correlated with dynamic standing balance in patients with ACL deficiency. Ten young men with unilateral ACL deficiency participated in this study. The mean time interval from the injury to the study was 12.8 months. Knee laxity measurements, passive re-positioning (PRP) and threshold for detection of passive motion (TTDPM) proprioception tests, quadriceps and hamstring muscle strength tests, and dynamic single-limb balance tests were performed for both injured and uninjured limbs. Significant differences between the injured and uninjured sides were observed for all test parameters. As independent variables, knee laxity, PRP proprioception, and muscle strength did not correlate with dynamic standing balance for the injured limb. However, a significant positive correlation (P<0.05) between TTDPM proprioception and dynamic single-limb stance balance was observed for the injured limb. To improve dynamic single-limb stance balance in patients with ACL injuries, training in TTDPM proprioceptive ability is recommended as the most important initial approach for such patients.

A novel method for measuring electromechanical delay of the vastus medialis obliquus and vastus lateralis.

Electromechanical delay (EMD) of the vastus medialis obliquus (VMO) and vastus lateralis (VL) is determined by measuring the interval between the time of onset of muscle activities and the time of onset of mechanical output. However, individual mechanical output of the VMO or the VL cannot be obtained with the conventional method because of the knee extension force as the mechanical output. Therefore, the objective of the present study was to develop a new method for measuring EMD of the VMO and VL individually. Twelve healthy volunteers participated in the experiment. The motor point of the target muscle was electrically stimulated to evoke a muscle twitch. Simultaneously, the electrical stimulation signal was transmitted to ultrasound apparatus via the electrocardiography input channel. The ultrasound apparatus was used to capture the patellar movement elicited by the muscle twitch. EMD was measured from the onset of the electrical stimulation to the onset of patellar movement. The results showed that the intraclass correlation coefficients for the reproducibility of the EMD measurements of the VMO and VL were greater than 0.8. The EMDs of the VMO and VL were 18.3 +/- 2.2 ms and 24.8 +/- 5.8 ms, respectively. This new method provides a more precise measurement of EMD in the VMO and VL than does the conventional method because of the use of patellar movement as the mechanical output.

Specific complications of the mobile-bearing total knee prosthesis.

Total knee arthroplasty (TKA) is considered as a standard procedure for the treatment of the severe degenerative knee joint. The outcomes of TKA have improved over the last few decades because of a better understanding of design features and surgical techniques. Contemporary tibial component designs in TKA can be subdivided into two groups (fixed or mobile-bearing) on the basis of different fundamental principles. The mobile-bearing knee is designed with better biomechanical concepts than the fixed-bearing knee, in terms of providing congruity and mobility of tibial bearing. It is expected that the mobile-bearing knee could resolve the problems of component wear and component loosening as seen in fixed-bearing knees. However, the long-term durability of mobile-bearing knees has not been extensively proven to have better outcomes than that of fixed-bearing knees. There are some potential clinical complications of mobile-bearing knees that must be addressed. On the basis of our previous clinical experience and a retrieval analysis, this article summarizes five specific complications including (1) early or late dislocation of the mobile-bearing component, (2) failure or dislodging of the meniscal bearing, (3) hemarthrosis associated with a failure of the meniscal-bearing knee, (4) failure of the patellar component, and (5) higher prevalence of osteolysis. Before taking advantage of mobile bearing, we should consider these specific clinical complications, which were not found in the fixed-bearing TKA.

Investigation of clinical effects of high- and low-resistance training for patients with knee osteoarthritis: a randomized controlled trial.

BACKGROUND AND PURPOSE: Muscle strength training is important for people with knee osteoarthritis (OA). High-resistance exercise has been demonstrated to be more beneficial than low-resistance exercise for young subjects. The purpose of this study was to compare the effects of high- and low-resistance strength training in elderly subjects with knee OA. SUBJECTS AND METHODS: One hundred two subjects were randomly assigned to groups that received 8 weeks of high-resistance exercise (HR group), 8 weeks of low-resistance exercise (LR group), or no exercise (control group). Pain, function, walking time, and muscle torque were examined before and after intervention. RESULTS: Significant improvement for all measures was observed in both exercise groups. There was no significant difference in any measures between HR and LR groups. However, based on effect size between exercise and control groups, the HR group improved more than the LR group. DISCUSSION AND CONCLUSION: Both high- and low-resistance strength training significantly improved clinical effects in this study. The effects of high-resistance strength training appear to be larger than those of low-resistance strength training for people with mild to moderate knee OA, although the differences between the HR and LR groups were not statistically significant.

Mobile-bearing knees reduce rotational asymmetric wear.

Polyethylene wear of bearing components is the most common long-term complication in total knee arthroplasty. One would anticipate differing kinematics would generate different wear patterns (including wear type, degree, and symmetry) on the articulating surface of mobile-bearing and fixed-bearing inserts. Because mobile-bearing designs facilitate movement of the insert relative to the tray when the knee rotates, we hypothesized mobile-bearing designs would reduce the incidence of rotational asymmetric wear. We examined 51 worn tibial inserts, including 15 from mobile-bearing rotating-platform posterior-cruciate-sacrificing dished prostheses and 36 from fixed-bearing posterior-cruciate-retaining flat prostheses, which were retrieved at revision surgery with an average implantation time of 115 months. We divided wear types into low-grade wear (burnishing, abrasion, and cold flow) and high-grade wear (scratching, pitting, metal embedding, and delamination) to assess wear degree of polyethylene. To assess symmetry of wear, the insert surface was divided into medial and lateral sides and each side was further divided into three equal zones along the anteroposterior direction. Low-grade wear was more common in mobile-bearing knees, whereas high-grade wear was more common in fixed-bearing knees. We identified no internal/external rotational asymmetric wear or anteroposterior asymmetric wear in mobile-bearing knees.

Stress analysis of the anterior tibial post in posterior stabilized knee prostheses.

Recent retrieval studies have indicated a high incidence of polyethylene wear on the anterior tibial post caused by impingement. This study investigated the influences of post-cam design features and component alignment on the stress distribution in the anterior tibial post when subjected to the impingement loading. Two three-dimensional finite element models of posterior stabilized knee prostheses were constructed, one with flat on flat (FF) and another with curve on curve (CC) contact surfaces between anterior tibial post and femoral cam. The polyethylene insert was modeled with elastoplastic properties. Nine cases, three hyperextension angles (0 degrees , 5 degrees , and 10 degrees ) combined with three axial tibial rotations (0 degrees , 2.5 degrees , and 5 degrees ) simulating different component alignments were analyzed. A vertical compressive load of 2,000 N and an extension moment of 45 Nm were applied simultaneously. The FF model had larger stress increases than the CC model in both hyperextension and tibial rotation compared with the neutral position. The maximum increase for the FF model was 68% in peak contact stress, 125% in von Mises stress, and 58% in tensile stress in the extreme case of 10 degrees of hyperextension combined with 5 degrees of axial rotation. Stress concentration was found at the anterior corner of the post in the FF model; this was not found in the CC model. The curve on curve design can reduce edge loading on the tibial post, especially during axial tibiofemoral rotation.

Fixed or mobile-bearing total knee arthroplasty.

Fixed and mobile-bearing in total knee arthroplasty are still discussed controversially. In this article, biomechanical and clinical aspects in both fixed and mobile-bearing designs were reviewed. In biomechanical aspect, the mobile-bearing design has proved to provide less tibiofemoral contact stresses under tibiofemoral malalignment conditions. It also provides less wear rate in in-vitro simulator test. Patients with posterior stabilized mobile-bearing knees had more axial tibiofemoral rotation than patients with posterior stabilized fixed-bearing knees during gait as well as in a deep knee-bend activity. However, in clinical aspect, the mid-term or long-term survivorship of mobile-bearing knees has no superiority over that of fixed-bearing knees. The theoretical advantages for mobile-bearing design to provide a long-term durability have not been demonstrated by any outcome studies. Finally, the fixed-bearing design with all-polyethylene tibial component is suggested for relatively inactive, elder people. The mobile-bearing design is suggested for younger or higher-demand patients due to the potential for reduced polyethylene wear and more normal kinematics response after joint replacement. For younger surgeon, the fixed-bearing design is suggested due to less demand for surgical technique. For experienced surgeon, one familiar surgical protocol and instrumentation is suggested rather than implant design, either fixed-bearing or mobile-bearing.

The accuracy of posterior condylar angles measured by one MR image.

The posterior condylar axis is used to establish femoral rotational alignment during total knee arthroplasty. One magnetic resonance image or computed tomography image often is used to determine the posterior condylar angle. However, the bony landmarks for determining the posterior condylar angle are not always present or obvious. We hypothesized measuring the posterior condylar angle on one slice would be no different when compared with using superimposed images with bony landmarks accurately identified in one or more slices. We analyzed magnetic resonance images of 95 knees without any bone disorders. Only 20 knees had all four bony landmarks apparent on any single slice. We compared the posterior condylar angles measured from one slice in which the medial sulcus, lateral epicondyle, or both posterior condyles were accurately identified with the angle measured from superimposed images. The average posterior condylar angle in the remaining 75 knees measured from superimposed images was 4.27 degrees +/- 1.95 degrees, which was similar to the angle measured from one in which the lateral epicondyle was the most accurately identified (3.95 degrees +/- 1.93 degrees). However, the largest difference (-3.53 degrees and 4.29 degrees) between three-dimensional and two-dimensional posterior condylar angle measurements in the worse-case scenario was unacceptable in clinical practice. We could not confirm our hypothesis: using one two-dimensional image to measure the posterior condyle angle is not accurate enough to determine its three-dimensional angle.

Influence of post-cam design on stresses on posterior-stabilized tibial posts.

Polyethylene wear and fracture at the tibial post of posterior- stabilized knee prostheses are common complications. Analyzing the stresses on various designs of tibial posts should predict the risk of damage. The post-cam of knee prostheses generally are designed either as flat-on-flat or curve-on- curve contact surfaces. We investigated stress on such tibial posts using a finite element method. We applied a 500-N anteroposterior shear force to the knee at neutral contact and 10 degrees tibial internal rotation when the knee was flexed at 60 degrees , 90 degrees , 120 degrees , and 150 degrees . There was more stress during tibial rotation compared with during neutral contact. The greatest increments of maximum von Mises stress and contact stress in the flat-on-flat model were from 20.4 to 46.1 MPa (126.3%) and from 22.2 to 55.7 MPa (151%), respectively, whereas in the curve-on-curve design they were from 20.5 to 22.7 MPa (10.7%) and from 33.0 to 35.4 MPa (7.2%), respectively. Increased stress concentration at the tibial post occurred at increased flexion angles along with axial tibial rotation. The curve-on-curve design reduced stress concentration when the knee sustained an anteroposterior shear force with tibial rotation.

Morphometrical measurements of resected surface of femurs in Chinese knees: correlation to the sizing of current femoral implants.

Morphometrical data were measured in the resected femurs of seventy Chinese patients who underwent total knee arthroplasties. Two measured parameters, the anterior-posterior length of the lateral condyle and the total width of the distal condyle, were compared to the anterior-posterior length and the medial-lateral width of five femoral implants currently used in Taiwan. Three implants (Duracon, NexGen and UKnee) have a larger medial-lateral width than the total width of the resected distal condyle for a given femoral implant anterior-posterior length. These implants tend to overhang the medial-lateral width of resected femurs from Chinese patients. In addition, one femoral implant (Duracon), which has previously been shown to be suitable for use in Caucasian patients, is not suitable in Chinese patients. Our results will allow manufacturers to design femoral implants better suited to Chinese patients.

Fatigue resistance analysis of tibial baseplate in total knee prosthesis--an in vitro biomechanical study.

BACKGROUND: Tibial baseplates were occasionally reported with clinical fatigue failures. This study postulated that tibial baseplate of a specific mobile bearing design with a uniform thickness across the baseplate offers more fatigue resistance than the fixed-bearing design. Tibial baseplates of a fixed bearing and a mobile bearing design were fatigue-tested in vitro to study their fatigue resistance. METHODS: Five samples of each design were tested under a sinusoidal loading between 90 N and 900 N at 30 Hz till failure or 10 million cycles. Experimental setup followed a standard published test method. Scanning electron microscope was used for inspecting the fracture surface of the failed baseplate. FINDINGS: Two baseplates of fixed bearing design failed before 10 million cycles. Fatigue crack advancement marks were visible on the fractured surface of the failed samples. The fractured cross-section showed that the failure started near the end of the fin, it was likely due to the stress concentration as stress singularity existed at a point of sudden geometrical change. Five mobile bearing baseplates passed the test. Design of the tibial baseplate without fin structure and with a uniform thickness across the whole baseplate could help reducing the incidence of fatigue failure. INTERPRETATION: The prosthesis survival rate was influenced by the long-term integrity of the metallic part of the prostheses such as the tibial baseplate. This study revealed that the tibial baseplate of a mobile bearing design with a uniform thickness provided better fatigue resistance than fixed bearing one. Standardized fatigue screening of the tibial baseplate was considered important in designing knee prostheses.

Polyethylene failure of the patellar component in New Jersey low-contact stress total knee arthroplasties.

In a postoperative 5- to 12-year follow-up study of 598 New Jersey low-contact stress total knee arthroplasties, 32 required revision. All retrieved patellar components were examined for polyethylene damage. These 32 cases had 3 types of failure: split rupture (7 cases), peripheral wear (21), and cantilevering breakage (4), respectively 75%, 64.3%, and 100% of which showed subluxation and/or tilting of the patellar component on the prerevision roentgenograms. Misalignment at the joint contact surfaces and rotational blockage of the mobile patellar component were considered the major causes of the failure. A design of a flatter metallic button (giving larger focal thickness of the polyethylene) and a dome-shaped polyethylene (reducing stress concentration at the pinnacle) may alleviate the failure driving mechanism should subluxation or tilting of the patellar component take place.

Evaluation of shoulder proprioception following muscle fatigue.

OBJECTIVE: To investigate the effect of shoulder muscle fatigue on glenohumeral proprioception. DESIGN: A repeated proprioception test was performed. BACKGROUND: The role of conditioning and fatigue in sport injuries remains controversial. It has been hypothesized that proprioceptive information plays an important role in joint stabilization and that muscle fatigue may alter proprioceptive ability. However, the effect of shoulder muscle fatigue on glenohumeral proprioception is still controversial. METHODS: Eleven normal subjects (mean age 27.3 years) participated in this study. Proprioception tests (on the dominant shoulder) were performed in which proprioception of the active reproduced and passive reproduced shoulder position was measured using an isokinetic dynamometer and a proprioception testing apparatus, respectively. The speed of active repositioning was at 2 deg/s and passive repositioning was at 0.5 deg/s. The mean value of maximum voluntary contraction and the number of repetitions for muscle fatigue were recorded. Post-fatigue proprioception test was started within 3 min after muscle fatigue. RESULTS: There was no significant difference of shoulder proprioception between pre- and post-fatigue determinations of passive repositioning in shoulder internal rotation, passive repositioning in external rotation and active repositioning in internal rotation. There was a significant difference between pre- and post-fatigue determination of active repositioning in external shoulder rotation (mean degrees: 2.57 (SD 1.02) vs. 4.96 (SD 1.73), P<0.05). CONCLUSION: Shoulder proprioception in active repositioning in external rotation was major affected by muscle mechanoreceptors in the presence of muscle fatigue. RELEVANCE: This study revealed that the shoulder proprioception after muscle fatigue in active repositioning in shoulder external rotation was affected more sensitively by the muscle mechanoreceptors than the joint mechanoreceptors. Increasing resistance of muscle fatigue would increase the shoulder proprioceptive ability.

The influence of surgical malalignment on the contact pressures of fixed and mobile bearing knee prostheses--a biomechanical study.

OBJECTIVE: To investigate the effect of surgical malalignment on contact pressures of fixed and mobile bearing knee prostheses. DESIGN: An experimental set-up was used to measure contact pressure on the tibial component of fixed and mobile bearing knee prostheses subjected to a compression load and surgical malalignment situations were simulated. BACKGROUND: Mobile bearing knee prostheses were designed to decrease tibiofemoral contact pressure by providing both high congruity and mobility. It was also assumed to accommodate surgical malalignment. However, few studies have reported the effect of malalignment of the tibiofemoral joint on contact pressure of fixed and mobile bearing knee prostheses. METHODS: Surgical malalignment situations were simulated to evaluate contact characteristics of tibial component of fixed and mobile bearing knee prostheses. The simulated malalignment conditions include the medial-lateral translation (0.5 and 1 mm), anterior-posterior translation (2 and 4 mm) and internal-external rotation (1 degrees, 3 degrees, 5 degrees and 10 degrees ) of the femoral component relative to the tibial component. Fuji pressure sensitive film was used to measure the contact pressure. RESULTS: The greatest increase of maximum contact pressure in the anterior-posterior maltranslation was 7.63% and 7.62% relative to the neutral contact situation in the fixed and mobile bearing designs respectively. In the medial-lateral maltranslation, there was 23.3% in the fixed bearing design and was 22.0% in the mobile bearing design. In the internal/external malrotation, the greatest increase of maximum contact pressure in the fixed bearing design was 27.1%, which was much higher than the mobile bearing design (22.4%). CONCLUSIONS: The mobile bearing design can reduce maximum contact pressure more significantly than the fixed bearing design when malalignment conditions of the tibiofemoral joint occurs, especially in the internal/external malrotation. The mobile bearing design offers the advantage of self-adjusting over the fixed bearing design to accommodate surgical malalignment. RELEVANCE: This study revealed that the mobile bearing design has smaller maximum contact pressures than the fixed bearing design in knee prosthesis under malalignment biomechanical tests. This result indicates that there is an advantage for a mobile bearing design over a fixed bearing design to accommodate malalignment conditions caused by surgical technique or soft tissues imbalance in total knee arthroplasty.

Late dislocation of rotating platform in New Jersey Low-Contact Stress knee prosthesis.

Five patients with late rotational dislocation of the rotating platform bearing in the New Jersey Low-Contact Stress total knee arthroplasty are reported. The prostheses had functioned well for 8 to 12 years before failure. Preoperative radiographs showed asymmetric femorotibial joint spaces. Entrapment of the dislocated bearing in three patients and spontaneous reduction of the dislocated bearing in another two patients were seen at revision. Femorotibial ligamentous instability was found after reduction. The retrieved polyethylene bearings showed advanced wear and cold flow deformities and the thickness was reduced. The revision arthroplasty was accomplished by replacement with a thicker bearing element. Progressive femorotibial ligament laxity and reduction of the thickness of polyethylene with wearing break down the originally well-balanced soft tissue tension of the knee. The rotational degree of the rotating platform bearing is unrestricted, which may result in late dislocation. Polyethylene wear is unavoidable in knee prostheses using metal contact with polyethylene even with a mobile-bearing design. Efforts to reduce polyethylene wear are mandatory.