Intraoperative assessment of midflexion laxity in total knee prosthesis

BackgroundSoft-tissue balancing of the knee is fundamental to the success of a total knee arthroplasty (TKA). In posterior-stabilized TKA, there is no stabilizer of the anterior–posterior translation in the midflexion range in which the cam-post mechanism does not engage yet. Therefore, instability in the midflexion range is suspected to occur in posterior-stabilized TKA. The purpose of this study was to measure the joint gap throughout a full range of motion and to analyze the joint gap laxity in the midflexion range after implantation of a mobile-bearing posterior-stabilized total knee prosthesis.MethodsJoint gap kinematics in 259 knees with varus osteoarthritis were measured during TKAs using a tensor device with the same shape of a total knee prosthesis of the same design was used. After the implantation of a mobile-bearing posterior-stabilized prosthesis and the reduction of the patellofemoral joint, the joint gap was measured at 0°, 30°, 60°, 90°, 120°, and 145° of flexion.ResultsThe center size of the joint gap was tight in extension and deep flexion and loose at midflexion ranges, especially at 30° of flexion (p < 0.001). The symmetry of the joint gap was varus at 0° and 145° of flexion (p < 0.001).ConclusionsOur results showed the joint gap laxity in the midflexion range after the implantation of a mobile-bearing posterior-stabilized prosthesis. Our new tensor device, which can attach the polyethylene insert trial, will provide the important information about the joint gap kinematics after implantation of total knee prostheses.Level of evidenceIV.     

Assessment of isometricity before and after total knee arthroplasty: A cadaver study

Total knee arthroplasty (TKA) relies on soft tissue to regulate joint stability after surgery. In practice, the exact balance of the gaps can be difficult to measure, and various methods including intra-operative spreaders or distraction devices have been proposed. While individual ligament strain patterns have been measured, no data exist on the isometricity of the soft tissue envelope as a whole. In this study, a novel device was developed and validated to compare isometricity in the entire soft tissue envelope for both the intact and TKA knee.A spring-loaded rod was inserted in six cadaver knee joints between the tibial shaft and the tibial plateau or tibial tray after removing a 7 mm slice of bone. The displacement of the rod during passive flexion represented variation in tissue tension around the joint. The rod position in the intact knee remained within 1 mm of its initial position between 15° and 135° of flexion, and within 2 mm (± 1.2 mm) throughout the entire range of motion (0–150°). After insertion of a mobile-bearing TKA, the rod was displaced a mean of 6 mm at 150° (p < 0.001). The results were validated using a force transducer implanted in the tibial baseplate of the TKA, which showed increased tibiofemoral force in the parts of the flexion range where the rod was most displaced. The force measurements were highly correlated with the displacement pattern of the spring-loaded rod (r = − 0.338; p = 0.006).A simple device has been validated to measure isometricity in the soft tissue envelope around the knee joint. Isometricity measurements may be used in the future to improve implantation techniques during TKA surgery.     

In vivo measurement of volumetric wear of a total knee replacement

A new Roentgen Stereophotogrammetric Analysis (RSA) system is reported; it can measure penetration of the metallic femoral component of a Total Knee Replacement (TKR) prosthesis into the polyethylene bearing on the tibial component. This system was used to analyse a study group of six Anatomic Graduated Components (AGC) knee prostheses more than 6 years post-implantation, and to compare with a control group of six newly implanted AGC prostheses. The volumetric loss of polyethylene was estimated by imaging each prosthesis at a series of different knee flexion angles. The mean difference between the RSA measured polyethylene bearing thickness and the manufacturer's quoted values for the control group was -0.03 mm (SD 0.17). The estimated linear penetration at 6.4 years in this prosthesis was determined to be 0.1 mm/year. Volumetric wear was estimated to be 600-700 mm(3)/year at 6.4 years, equating to approximately 100 mm(3)/year. This does not appear to be clinically significant amount of wear as this prosthesis has excellent survival at 10 years.     

Modern femoral component design in total knee arthroplasty shows a lower patellar contact force during knee flexion compared with its predecessor

BackgroundThe relationship between the femoral component design in total knee arthroplasty (TKA) and the patellofemoral contact force, as well as the soft tissue balance, has not been well reported thus far.MethodsTwenty-eight mobile-bearing posterior-stabilized (PS) TKAs using the traditional model (PFC Sigma) and 27 mobile-bearing PS TKAs using the latest model (Attune) were included. Surgeries were performed using the measured resection technique assisted with the computed tomography (CT)-based free-hand navigation system. After all the trial components were placed, patellar contact forces on the medial and lateral sides were measured using two uniaxial ultrathin force transducers with the knee at 0°, 10°, 30°, 60°, 90°, 120°, and 135° of flexion. The joint component gap and the varus ligament balance of the femorotibial joint were also measured. The non-paired Student’s t-test was conducted to compare the values of the two groups.ResultsThe medial patellar contact force was significantly lower for Attune group than for PFC Sigma group at 120° of knee flexion (P = 0.0058). The lateral patellar contact force was also significantly lower for Attune group than PFC Sigma group at 120° and 135° of knee flexion (P = 0.0068 and P = 0.036). The joint component gap, as well as the varus ligament balance, showed no statistically significant difference between the two groups.ConclusionsReduced thickness and width of the anterior flange of the femoral component in the Attune may play a role in low patellar contact force.     

Numerical analysis of variations in posterior cruciate ligament properties and balancing techniques on total knee arthroplasty loading

Total knee arthroplasty (TKA) is a widely used and successful orthopaedic procedure. During TKA, the posterior cruciate ligament (PCL) can either be retained or substituted by a post-cam mechanism. One of the main functions of the PCL is to facilitate femoral rollback during knee flexion. For adequate PCL functioning, the PCL should be balanced correctly after TKA. A tight PCL leads to more femoral rollback at the expense of a higher joint compression and potential polyethylene wear. Frequently used surgical techniques to balance a tight PCL are PCL release and increasing the posterior tibial slope. The objective of this study was to evaluate the effects of variations in PCL properties and balancing techniques on the mechanical outcome of a total knee replacement during a weight-bearing squatting movement (flexion range = 45–150°). For this purpose, a prosthetic finite element knee model was developed including a PCL having adjustable properties. Varying the PCL stiffness and PCL steepness (elevation angle) with respect to the tibial plateau considerably affected the TKA loading characteristics. Both a relatively high PCL stiffness and a low elevation angle at the start of the flexion cycle led to a high PCL force (1400–1500 N) and a high peak polyethylene contact stress of roughly 52 MPa during deeper knee flexion (120°). Releasing the PCL with roughly 4 mm or increasing the posterior tibial slope to 7° reduced the PCL force to 300–400 N and the polyethylene peak contact stress to 35–42 MPa at 120° of flexion. The femoral rollback patterns during deep knee flexion were only marginally affected when extra posterior tibial slope was added, whereas additional PCL release resulted in paradoxical anterior movement of the femur.     

Effect of voluntary soft tissue tension and articular conformity after total knee arthroplasty on in vivo anteroposterior displacement

The in vivo relationship between the degree of voluntary soft tissue tension and articular conformity after total knee arthroplasty (TKA) and anteroposterior (AP) displacement was simultaneously investigated by analyzing LCS prostheses (posterior cruciate ligament-sacrificing rotating platform design) in 20 knees from 20 patients. AP displacement was measured using the KT-2000 arthrometer, at 30° and 75° flexion, while patients were conscious and under anesthesia; 30° flexion was regarded as high conformity and 75° as low conformity. Mean displacements at 30° and 75° were 5.1 mm and 7.0 mm, respectively, in conscious patients, and 6.7 mm and 7.7 mm, respectively, in patients under anesthesia. AP displacement was significantly associated with soft tissue tension (p = 0.026) and conformity (p = 0.001). No interaction was observed between the two variables (p = 0.193). Surgeons should recognize that AP displacement is greater in anesthetized patients than in conscious patients, regardless of the degree of conformity, and that higher conformity shows less displacement, regardless of the degree of soft tissue tension. These results may help surgeons to determine the intra-operative AP displacement required for proper postoperative displacement in the current prosthetic design.     

Peri-prosthetic bone mineral density after simultaneous bilateral total knee arthroplasty under oral bisphosphonate therapy — A comparison between mobile- and fixed-bearing prostheses

BackgroundPeri-prosthetic bone mineral density (BMD) decreases after total knee arthroplasty (TKA). We aimed to specifically compare peri-prosthetic BMD changes between mobile- and fixed-bearing prostheses in patients undergoing oral bisphosphonate therapy, hypothesizing that mobile-bearing components would have a favorable effect on postoperative peri-prosthetic BMD.MethodsThis prospective cohort study investigated 30 patients who underwent simultaneous bilateral TKA for primary knee osteoarthritis between December 2007 and September 2012. All patients underwent mobile-bearing TKA in one knee and fixed-bearing TKA in the other and received oral alendronate therapy at a dosage of 35 mg/week. Peri-prosthetic and lumbar spine BMDs were measured using dual X-ray absorptiometry scans, with peri-prosthetic BMD changes being compared between the two prostheses in each patient at six months and one, two, three, and five years post-operation.ResultsClinical results did not significantly differ between two prostheses. Relative change of lumbar spine BMD was significantly greater at five years post-operation than at one year post-operation (p = 0.01), and was significantly correlated at five years post-operation with peri-prosthetic BMD in the central femur (r = 0.39, p = 0.002), posterior femur (r = 0.39, p = 0.002), and medial tibia (r = 0.42, p = 0.007).ConclusionsThere was no difference in peri-prosthetic BMD changes between two prostheses in patients undergoing oral bisphosphonate therapy. Our results suggest that the influence of oral bisphosphonate therapy might offset the influence of prosthetic design. Thus, oral bisphosphonate therapy may be more effective than prosthetic design selection in preventing post-TKA peri-prosthetic BMD loss.Level of evidence: II     

高屈曲型与普通型假体人工全膝关节置换后的早期功能恢复比较

背景：高屈型假体理论上优于普通型假体,尤其是在置入后关节屈曲度方面。 目的：评价高屈曲型与普通型假体单侧人工全膝关节置换后早期的功能康复之间的差异。 方法：回顾性对照分析新疆医科大学第四附属医院骨科中心初次行人工全膝关节置换的133例患者141膝,植入假体分别为高屈曲型与普通型,比较两组患者置入后疗效。 结果与结论：置入后高屈组1例发生一过性腓总神经麻痹,1例置入后切口愈合不良,42 例44 膝获随访,失访2例3膝,随访12~19个月;普通型2例发生切口愈合不良,1例浅静脉血栓,86例91膝获随访,失访3例3膝,随访12~23 个月。置入后1年两组HSS 评分、疼痛评分、肌力评级比较,差异无显著性意义(P > 0.05)。高屈曲型在活动度、屈曲挛缩度方面优于普通型(P < 0.05)。说明高屈曲型假体单侧人工全膝关节置换早期在关节活动范围和屈曲挛缩度方面评分优于普通型。     

Comparison of intraoperative soft tissue balance measurement between two tensor systems in total knee arthroplasty

BackgroundDifferences in measurement outcomes using different tensors in total knee arthroplasty (TKA) remain unknown. This study aimed to compare intraoperative soft tissue balance between two measurement systems in TKA.MethodsThis study included 24 cruciate-retaining (CR) and 27 posterior-stabilized (PS) TKAs for varus-type osteoarthritis. All TKAs were performed with the measured resection technique. Intraoperative soft tissue balance was assessed with femoral component placement and patellofemoral joint reduction throughout the range of motion. Measurements were performed using the Offset Repo-Tensor (Zimmer) and Ligament Sensor Tensor (Depuy). Joint component gaps and varus/valgus ligament balance were compared statistically in CR and PS TKAs.ResultsJoint component gaps with the Offset Repo-Tensor were significantly larger at 0°, 10°, 30°, and 60° of knee flexion in CR TKA and at all knee flexion angles, except 0° and 135°, in PS TKA than those with the Ligament Sensor Tensor (P < .05). Varus balance with the Offset Repo-Tensor was significantly larger at 0°, 10°, and 30° of knee flexion in CR TKA and at 10°, 30°, 60°, 90°, and 120° of knee flexion in PS TKA than that with the Ligament Sensor Tensor (P < .05).ConclusionSurgeons should consider the discrepancy in assessing intraoperative soft tissue balance depending on the tensor used. The Ligament Sensor Tensor tended to underestimate soft tissue balance in CR and PS TKAs compared with the Offset Repo-Tensor, especially in PS TKA. Thus, surgeons should know the difference in values between the offset-type tensor with consistent distraction force and a spring-loaded tensor.     

Introduction of a new mobile-bearing total knee prosthesis: minimum three year follow-up of an RCT comparing it with a fixed-bearing device

As part of the step-wise validation of a new prosthesis (TMK), we previously published the 1 year results of a randomised controlled trial in patients undergoing bilateral knee replacement [Price A., Rees J., Beard D., Juszczak E. et al. A mobile-bearing total knee prosthesis compared with a fixed-bearing prosthesis. JBJS B 2003;85-B-1:62-7.]. Forty patients had the new mobile-bearing prosthesis implanted in one knee and an established fixed-bearing device in the other (AGC). We now report the 3 year status of these patients and, in addition, review a separate multi-centre cohort of 172 patients who had undergone unilateral arthroplasty with the TMK.

No significant differences were found in outcome (American Knee Society Score and Oxford Knee Score) between the two prostheses. The greater incidence of “clicking” in the mobile-bearing knee, reported in the previous review, persisted (TMK = 48%, AGC = 30%). The presence of this mechanical noise was found to have no relationship with outcome in either of the prostheses. The unilateral cohort study showed an acceptable complication rate for the new prosthesis, although some patients reported subjective instability.

The method of controlled introduction of the TMK, of which this constitutes a further step, has allowed us to assess the significance of a reported problem (clicking) and to provide scientific data from which other surgeons can decide about use of the implant.     

Rotational kinematics differ between mild and severe valgus knees in total knee arthroplasty

BackgroundThere is no consensus regarding femorotibial rotational kinematics in total knee arthroplasty (TKA) for valgus knee deformity. Additionally, whether the degree of valgus deformity influences intraoperative rotational kinematics and postoperative clinical scores remains unclear. The objectives of this study were to investigate whether the valgus angle is associated with intraoperative rotational kinematics in TKA for valgus knee deformity and to examine the relationship between rotational kinematics and postoperative clinical results.Materials and methodsA total of 24 knees with valgus deformity for TKA were included in this study and were divided into two groups depending on the femorotibial angle (FTA); there were 11 knees in the severe valgus group (FTA < 160°) and 13 knees in the mild valgus group (FTA ≥ 160°). Intraoperative femorotibial rotational kinematics from knee extension to flexion were evaluated using an image-free navigation system and postoperative clinical results (range of motion and subjective outcomes) were evaluated 1 year postoperatively. All parameters were compared between the two groups.ResultMild valgus knee showed tibial internal rotation during knee flexion before implantation, whereas severe valgus knee showed tibial external rotation during knee flexion before implantation. The postoperative flexion angle was positively correlated with the tibial internal rotation angle after implantation in the mild valgus group only.ConclusionIntraoperative rotational kinematics before implantation differed between mild and severe valgus knee deformity in TKA. Intraoperative tibial rotation influenced the postoperative knee flexion angle in mild, but not severe, valgus knee deformity. Ideal postoperative rotational kinematics may be different between the two groups and the difference may be taken into consideration in implant selections and surgical techniques.     

The medial gap is a reliable indicator for intraoperative soft tissue balancing in posterior-stabilized total knee arthroplasty

BackgroundAppropriate soft tissue balance and accurate alignment are important for successful total knee arthroplasty (TKA). However, the optimal technique for establishing and measuring soft tissue balancing remains unclear. The aim of this study was to analyze the intraoperative medial and lateral gap pattern using digital knee balancer in posterior-stabilized (PS) TKA.MethodsThis study involved 55 patients with medial osteoarthritis who underwent a primary TKA using an image-free navigation system. The extension gap and the flexion gap at 90° knee flexion were assessed using an offset seesaw-type digital balancer. Continuous joint distraction force from 10 lb to 60 lb was applied. Medial gap, lateral gap, and varus angle were measured.ResultsThe medial bone gap difference between extension and flexion was constant regardless of the distraction force from 20 lb to 60 lb. The lateral bone gap was significantly greater than the medial bone gap in extension and flexion from 30 lb to 60 lb (P < 0.05). The varus angle changed depending on the distraction force, especially in flexion. The varus angle in flexion was significantly greater than that in extension from 40 lb to 60 lb (P < 0.05).ConclusionsThe medial bone gap is a reliable indicator unaffected by the distraction force during surgery and is useful for adjusting the medial gap in extension and flexion appropriately to ensure medial stability in PS-TKA. The digital knee balancer and navigation system support both precise gap assessment and surgery.     

Retropatellar contact characteristics before and after total knee arthroplasty

PURPOSE: Qualitative analysis of the retropatellar contact characteristics after total knee arthroplasty. MATERIAL AND METHODS: Six cadaveric knees were investigated before and after implantation of a Genesis I knee prosthesis without and with patellar resurfacing in different positions. Joint contact characteristics were evaluated with Fuji Prescale film type 'Super Low' and analyzed qualitatively in nine quadrants. The pressure was determined from a 5-s loading duration in four different knee positions between 45 degrees and 120 degrees of flexion. The femoral component of the prosthesis was implanted in neutral as well as in internal and external rotation. A quadriceps force of 280 N with either a predominant medial or lateral pulling direction was applied. RESULTS: Without prosthesis the largest contact area is between 60 degrees and 90 degrees of flexion. A lateral muscle force direction as well as an external rotation increased the frequency of loading in the medial quadrants. After implantation of the prosthesis the central and superior quadrants were predominantly contacted irrespective of the flexion angle. No marked differences between the flexion angles were found. Implantation of the patellar resurfacing led to contact in the three central quadrants. CONCLUSION: Implantation of the endoprosthesis leads to increased contact in slight and extreme flexion angles. Especially the central areas are increasingly loaded. No predominant influence of the rotation of the femoral component or the direction of the muscle pull was found. An improved distribution of the contact area could not be demonstrated after patellar resurfacing.     

Navigation-based analysis of associations between intraoperative joint gap and mediolateral laxity in total knee arthroplasty

BackgroundNo data have demonstrated how joint gap measured under a distraction force is actually associated with mediolateral laxity evaluated under a varus–valgus force during total knee arthroplasty (TKA). This study aimed to investigate the correlations between them using a navigation system.MethodsA total of 113 primary navigated TKAs were included. After bone resection and soft-tissue balancing, the component gap was measured with a distraction force of 60 N and 80 N for both the medial and lateral compartment (i.e. a total of 120 N and 160 N) at 0°, 10°, 30°, 60°, 90°, and 120° knee flexion. After the final prosthetic implantation and capsule closure, mediolateral laxity under a maximum varus–valgus stress was recorded with image-free navigation at each knee flexion angle. The correlation between joint gap laxity (total differences between component gap and insert thickness in the medial and lateral compartment) and mediolateral laxity was analyzed using Spearman’s rank correlation coefficient.ResultsThe joint gap laxity under both distraction forces showed significant positive correlations with mediolateral laxity at 10°, 30°, 60°, and 90° flexion, whereas no correlation was observed at extension and 120° flexion. The correlations were stronger in gap measurement under 80 N than 60 N at all examined ranges. In patients with body mass indexes (BMIs) ≥ 30 kg/m², the correlation became non-significant.ConclusionIntraoperative joint gap laxity was associated with mediolateral laxity after TKA, especially at mid-flexion angles. The factors weakening the correlations were a lower applied distraction force for gap measurement and a larger BMI.     

Three-dimensional kinematics during deep-flexion kneeling in mobile-bearing total knee arthroplasty

We performed an in vivo radiographic analysis of tibiofemoral and polyethylene (PE) insert motions during weight-bearing kneeling beyond 120° of flexion in one high-flexion knee arthroplasty design to determine if kinematics changed over time and if axial rotation occur between the PE insert and the tibial baseplate. Twenty knees implanted with a posterior-stabilized rotating-platform (RP) knee arthroplasty were postoperatively evaluated at 3, 6, and 12 months. The averaged flexion angles were 122°, 129°, and 131° at 3, 6, and 12 months, respectively, showing that the improvement of flexion was achieved up to 6 months. The femoral condyles translated posteriorly from extension to maximum flexion. There was a significant increase in AP translation of femoral lateral condyle in the maximum flexion kneeling between 12 months and the two other intervals (p = 0.0003 at 3 months and p = 0.016 at 6 months), while no differences in those of medial condyle between all intervals. Almost all rotation occurred at the surface between the tibial baseplate and the PE insert (p = 0.0479 at 3 months, p = 0.0008 at 6 months, and p = 0.0479 at 12 months), almost no rotation occurred at the surface between the PE insert and the femoral component. There were significant increases in the amount of internal rotation angle during full flexion between the tibial component and the PE insert up to 12 months. Knees implanted with this RP knee arthroplasty design show deep-flexion knee kinematics that are consistent with the implant design intent.     

Preoperative varus deformity of the knee affects the intraoperative joint gap in unicompartmental knee arthroplasty

BackgroundIt is generally believed that contraction of the soft tissue structures on the medial side may occur in the knee with severe varus deformity. However, the relationship between the severity of varus deformity of the knee and the intraoperative soft tissue balance in unicompartmental knee arthroplasty (UKA) has not been well reported thus far.MethodsOne hundred and three consecutive medial UKAs were enrolled. After the femoral trial prosthesis was placed, the component gap was measured at 10° (extension) and 120° (flexion) of flexion using a UKA tensor. The pre-osteotomy gap was then calculated from the thickness of the bone cut. Paired Student’s t-test was used to compare the component gap, as well as the pre-osteotomy gap, in extension and those in flexion. The relationship between the preoperative Hip-Knee-Ankle (HKA) angle and the pre-osteotomy gap was analysed using Pearson’s correlation coefficient and simple linear regression analysis.ResultsThe component gap in extension was significantly smaller than that in flexion while the pre-osteotomy gap in extension was significantly wider than that in flexion. There was a positive correlation between the severity of varus deformation in preoperative knee and the pre-osteotomy gap in extension, while there was no correlation between the preoperative HKA angle and the pre-osteotomy gap in flexion.ConclusionsThe tension of the medial tightness does not correlate with the degree of preoperative varus deformity in UKA.     

Accuracy of implantation during computer-assisted minimally invasive Oxford unicompartmental knee arthroplasty: A comparison with a conventional instrumented technique

The prognosis of unicompartmental knee arthroplasty (UKA) is strongly associated with the accuracy of the component alignment. To determine the accuracy of navigated UKA during primary minimally invasive Oxford UKA, twenty-nine knees of 29 consecutive patients (Group A) implanted using conventional instrumented UKA were followed by 23 knees of 17 consecutive patients (Group B) implanted by navigation assisted UKA and radiological results regarding alignments of the femorotibial mechanical axis, femur, and tibial component were compared in the two groups. Assessments of mechanical limb alignment revealed statistically significant increases in mechanical limb alignment post-operatively in both groups (p = 0.0 for both). In terms of component alignment, Group B had more prostheses implanted in the satisfactory range (> ± 3° from the targeted values) for the femoral and tibial components than Group A. There were no significant differences in the rate of prosthesis implanted within the range of radiographic alignment variations for the coronal implantation of either femoral or tibial components in both groups. (Radiographic alignment variation; coronal orientation of femoral components 90 ± 10°, sagittal orientation of femoral components 90 ± 5°, coronal orientation of tibial components from 10° varus to 5° valgus, sagittal orientation of tibial components from 7° of posterior tibial flexion to 5° of anterior tibial flexion). However, significant increases in the accuracies of sagittal implantation of femoral and tibial components were observed in Group B versus Group A. Our data suggest that navigated implantation improves the accuracy of the radiological implantation of the Oxford UKA prosthesis without increasing complications versus conventional UKA.     

Effect of knee laxity on in vivo kinematics of meniscal-bearing knee prostheses

INTRODUCTION: This study determined the relationship between the in vivo kinematics and joint laxity of mobile-bearing knees. MATERIALS AND METHODS: Ten subjects who were judged clinically successful after a posterior-cruciate-ligament-retaining mobile-bearing total knee arthroplasty were analyzed under dynamic conditions using fluoroscopy, following a quantitative evaluation of coronal and sagittal laxity using Telos and KT-2000 arthrometers under static conditions. RESULTS: Under static conditions, the anteroposterior laxity averaged 10.3 mm at 30 degrees of flexion, and 8.3 mm at 75 degrees . Abduction and adduction at full extension averaged 3.7 and 4.4 mm, respectively. Under dynamic conditions, 7/10 subjects demonstrated posterior femoral rollback of their lateral condyle moving from full extension to maximum knee flexion, and 8/10 experienced normal axial rotation patterns during deep knee bend activity. Furthermore, 7/10 subjects exhibited screw-home motion. 4/10 subjects experienced greater than 1.0 mm of condylar liftoff. CONCLUSION: We could not predict a consistent relationship between the laxity and kinematics. The activity of the muscles and ligamentous tension under dynamic conditions might have a much greater effect on the kinematics with currently used prosthetic designs than the laxity of ligamentous structures under static conditions.     

Influence of posterior condylar offset on knee flexion after cruciate-sacrificing mobile-bearing total knee replacement: A prospective analysis of 410 consecutive cases

The range of motion of the knee joint after Total Knee Replacement (TKR) is a factor of great importance that determines the postoperative function of patients. Much enthusiasm has been recently directed towards the posterior condylar offset with some authors reporting increasing postoperative knee flexion with increasing posterior condylar offset and others who did not report any significant association. Patients undergoing primary total knee replacement were included in a prospective multicentre study and the effect of the posterior condylar offset on the postoperative knee flexion was assessed after adjusting for known influential factors. All knees were implanted by three senior orthopedist surgeons with the same cemented cruciate-sacrificing mobile-bearing implant and with identical surgical technique. Clinical data, active knee flexion and posterior condylar offset were recorded preoperatively and postoperatively at a minimal one year follow-up for all patients. Univariate and multivariate linear models were fitted to select independent predictors of the postoperative knee flexion.Four hundred and ten consecutive total knee replacements (379 patients) were included in the study. The mean preoperative knee flexion was 112°. The mean condylar offset was 28.3 mm preoperatively and 29.4 mm postoperatively. The mean postoperative knee flexion was 108°. No correlation was found between the posterior condylar offset or the tibial slope and the postoperative knee flexion. The most significant predictive factor for postoperative flexion after posterior-stabilized TKR without PCL retention was the preoperative range of flexion, with a linear effect.     

Kinematic analysis of mobile-bearing and fixed-bearing knee prostheses by simulation

The object of the study was to model fixed-bearing knee prostheses (FBKs) and mobile-bearing knee prostheses (MBKs) during weight-bearing deep knee bends and to analyse and compare the kinematics of the two prosthesis types. To obtain quantitative data, an overall model of a leg was constructed, and this included a three-dimensional model of the tibiofemoral joint and simplified twodimensional models of the ankle and patellofemoral joint. The simulated movement pattern of the tibiofemoral contact point in the FBK was analysed to show the posterior contact position on the tibia at full extension and anterior translation as the knee was flexed from 30° to 90°. The simulated maximum displacements of the medial and lateral contact positions of the FBK were 5.6 and 6.2 mm, respectively. These results were almost in agreement with experimental studies. Compared with the FBK, the movement pattern of the tibiofemoral contact point in the MBK for the anterior contact position on the tibia at full extension and posterior translation, with respect to the tibia as the knee was flexed, gave results closer to those of the normal knee. The simulated displacements of the medial and lateral contact positions of the MBK with respect to the tibia were 9.0 and 13.0 mm from full extension to 90° flexion, respectively. The difference in the kinematic results between the FBK and the MBK could be accounted for by movement of the insert and the larger force of the posterior cruciate ligament on the MBK.