Tibiofemoral force following total knee arthroplasty: comparison of four prosthesis designs in vitro

Critical Examination of Methods to Determine Tibiofemoral Kinematics and Tibial Contact Kinematics Based on Analysis of Fluoroscopic Images

Goals of knee replacement surgery are to restore function and maximize implant longevity. To determine how well these goals are satisfied, tibial femoral kinematics and tibial contact kinematics are of interest. Tibiofemoral kinematics, which characterize function, is movement between the tibia and femur whereas tibial contact kinematics, which is relevant to implant wear, is movement of the location of contact by the femoral implant on the tibial articular surface. The purposes of this review article are to describe and critique relevant methods to guide correct implementation. For tibiofemoral kinematics, methods are categorized as those which determine (1) relative planar motions and (2) relative three-dimensional (3D) motions. Planar motions are determined by first finding anterior-posterior (A-P) positions of each femoral condyle relative to the tibia and tracking these positions during flexion. Of the lowest point (LP) and flexion facet center (FFC) methods, which are common, the lowest point method is preferred and the reasoning is explained. 3D motions are determined using the joint coordinate system (JCS) of Grood and Suntay. Previous applications of this JCS have resulted in motions which are largely in error due to "kinematic crosstalk." Requirements for minimizing kinematic crosstalk are outlined followed by an example, which demonstrates the method for identifying a JCS that minimizes kinematic crosstalk. Although kinematic crosstalk can be minimized, the need for a JCS to determine 3D motions is questionable based on anatomical constraints, which limit varus-valgus rotation and compression-distraction translation. Methods for analyzing tibial contact kinematics are summarized and validation of methods discussed.

Similar outcomes including maximum knee flexion between mobile bearing condylar-stabilised and fixed bearing posterior-stabilised prosthesis: a case control study

Purpose

Prosthesis design influences stability in total knee arthroplasty and may affect maximum knee flexion. Posterior-stabilised (PS) and condylar-stabilised (CS) designed prosthesis do not require a posterior-cruciate ligament to provide stability. The aim of the current study was to compare the range of motion (ROM) and clinical outcomes of patients undergoing cemented total knee arthroplasty (TKA) using either a PS or CS design prosthesis.

Methods

A total of 167 consecutive primary TKAs with a CS bearing (mobile deep-dish polyethylene) were retrospectively identified and compared to 332 primary TKA with a PS constraint, with similar design components from the same manufacturer. Passive ROM was assessed at last follow-up with use of a handheld goniometer. Clinical scores were assessed using Patient-Reported Outcome Measures (PROMs); International Knee Society (IKS) knee and function scores and satisfaction score. Radiographic assessment was performed pre and post operatively consisting of mechanical femorotibial angle (mFTA), femoral and tibial mechanical angles measured medially (FMA and TMA, respectively) on long leg radiographs, tibial slope and patella height as measured by the Blackburne-Peel index (BPI).

Results

Both groups had a mean follow-up of 3 years (range 2–3.7 years). Mean post-operative maximum knee flexion was 117° ± 4.9° in the PS group and 119° ± 5.2° in the CS group (p = 0.29). Postoperative IKS scores were significantly improved in both groups compared to preoperative scores (p < 0.01). The mean IKS score in the PS group was 170.9 ± 24.1 compared to 170.3 ± 22.5 in the CS group (p = 0.3). Both groups had similar radiographic outcomes as determined by coronal and sagittal alignment, tibial slope and posterior condylar offset ratio measurements. When considering the size of tibial slope change and posterior-condylar offset ratio, there was no differences between groups (p = 0.4 and 0.59 respectively). The PS group had more interventions for post-operative stiffness (arthrolysis or manipulation under anaesthesia) 8 (2.7%) compared to 1 (0.6%) in the CS group (p = 0.17).

Conclusion

Condylar-stabilised TKA have similar patient outcomes and ROM at a mean follow-up of 3 years compared to PS TKA. Highly congruent inserts could be used without compromising results in TKA at short term.

Level of evidence

Level IV, retrospective case control study.

Dislocation after Posterior Stabilized Primary Total Knee Replacement: A Rare Complication in Four Cases

Introduction

Dislocation of a total knee arthroplasty is a rare complication that has rarely been described, while the total knee arthroplasty is frequently performed. From literature, we know patient-related factors, like obesity, neuropsychiatric disease, and severe valgus or varus deformity, are associated with higher risk of dislocation. We show our cases for awareness of the risk factors for surgeons. Case Presentations. We present four patients with a dislocation after a total knee arthroplasty. We compare these case reports with previous literature and show the most important risk factors for these dislocations. In our cases, three of them suffered from obesity, which possibly has contributed to the dislocation. Three patients did have instability which emphasizes the importance of ligament balancing while performing a total knee replacement. In all cases, an exchange of the polyethylene liner was performed.

Conclusion

Implant-related factors and surgical technique as well as patient-related factors can contribute to this uncommon complication. Obesity, neuropsychiatric disorders, and a severe valgus or varus deformity are important patient-related risk factors. Our cases show these risk factors too. Some of these risk factors were encountered as well as other comorbidity factors. Such risk factors must be taken into consideration when deciding whether to perform a total knee arthroplasty. This stresses the importance of patient education and shared decision-making before performing a total knee replacement.

Sensor-Assisted Total Knee Arthroplasty: A Narrative Review

Wireless intraoperative load sensors have been used to improve the quality of soft-tissue balancing during total knee arthroplasty(TKA). Recent studies using the sensors have demonstrated reductions in gap imbalance, as well as early improvement of patient-reported clinical outcomes and low rates of arthrofibrosis. However, well-designed prospective studies are needed to determine whether the application of the sensor technology for TKA will have clinical benefits and improve the survival of prosthesis. Knowledge of the load-sensing technology (advantages and disadvantages, potential pitfalls, and future prediction) is crucial to apply this new TKA technique successfully. Herein, we conduct a narrative review of previous studies on this technique.

Quantified in vitro tibiofemoral contact during bodyweight back squats

Squats are a common lower extremity task used in strength and conditioning, balance training, and rehabilitation. It is important to understand how slight alterations in lower extremity kinematics during a squat affect the internal joint loading of the knee. This study directly quantified tibiofemoral contact throughout the in vitro simulation of a bodyweight back squat performed two ways: a heel squat (knees in line with toes) and a toe squat (knees anterior to the toes) at peak knee flexion. Three cadaveric right lower extremities were instrumented and positioned into the University of Texas Joint Load Simulator. Kinematics, kinetics, and predicted muscle forces from a 20-year-old athletic male performing the two back squats were used as inputs for the in vitro simulations. The quantified tibiofemoral contact area, peak pressure, net force, and center of pressure location were significantly different between squat types (p > 0.05). Net contact area on the tibial plateau at peak knee flexion was significantly larger in the heel versus toe squat (599 ± 80 mm² vs. 469 ± 125 mm²; p < 0.05). Peak lateral pressure was significantly higher in the heel versus toe squat (2.73 ± 0.54 MPa vs. 0.87 ± 0.56 MPa; p < 0.05). Results suggest the heel squat generates an even load distribution, which is less likely to affect joint degeneration. Future in vitro simulations should quantify the effects lower extremity kinematics, kinetics, and individual muscle forces have on tibiofemoral contact parameters during common athletic tasks.

An Improved Tibial Force Sensor to Compute Contact Forces and Contact Locations In Vitro After Total Knee Arthroplasty

Contact force imbalance and contact kinematics (i.e., motion of the contact location in each compartment during flexion) of the tibiofemoral joint are both important predictors of a patient's outcome following total knee arthroplasty (TKA). Previous tibial force sensors have limitations in that they either did not determine contact forces and contact locations independently in the medial and lateral compartments or only did so within restricted areas of the tibial insert, which prevented them from thoroughly evaluating contact force imbalance and contact kinematics in vitro. Accordingly, the primary objective of this study was to present the design and verification of an improved tibial force sensor which overcomes these limitations. The improved tibial force sensor consists of a modified tibial baseplate which houses independent medial and lateral arrays of three custom tension-compression transducers each. This sensor is interchangeable with a standard tibial component because it accommodates tibial articular surface inserts with a range of sizes and thicknesses. This sensor was verified by applying known loads at known locations over the entire surface of the tibial insert to determine the errors in the computed contact force and contact location in each compartment. The root-mean-square errors (RMSEs) in contact force are ≤ 6.1 N which is 1.4% of the 450 N full-scale output. The RMSEs in contact location are ≤ 1.6 mm. This improved tibial force sensor overcomes the limitations of the previous sensors and therefore should be useful for in vitro evaluation of new alignment goals, new surgical techniques, and new component designs in TKA.

Characterization and Correction of Errors in Computing Contact Location Between Curved Articular Surfaces: Application to Total Knee Arthroplasty

In total knee arthroplasty (TKA), one common metric used to evaluate innovations in component designs, methods of component alignment, and surgical techniques aimed at decreasing the high rate of patient-reported dissatisfaction is tibiofemoral contact kinematics. Tibiofemoral contact kinematics are determined based on the movement of the contact locations in the medial and lateral compartments of the tibia during knee flexion. A tibial force sensor is a useful instrument to determine the contact locations, because it can simultaneously determine contact forces and contact locations. Previous reports of tibial force sensors have neither characterized nor corrected errors in the computed contact location (i.e., center of pressure) between the femoral and tibial components in TKA that, based on a static analysis, are caused by the curved articular surface of the tibial component. The objectives were to experimentally characterize these errors and to develop and validate an error correction algorithm. The errors were characterized by calculating the difference between the errors in the computed contact locations when forces were applied normal to the tibial articular surface and those when forces were applied normal to the tibial baseplate. The algorithm generated error correction functions to minimize these errors and was validated by determining how much the error correction functions reduced the errors in the computed contact location caused by the curved articular surface. The curved articular surface primarily caused bias (i.e., average or systematic error) which ranged from 1.0 to 2.7 mm in regions of high curvature. The error correction functions reduced the bias in these regions to negligible levels ranging from 0.0 to 0.6 mm (p < 0.001). Bias in the computed contact locations caused by the curved articular surface of the tibial component as small as 1 mm needs to be accounted for, because it might inflate the computed internal-external rotation and anterior-posterior translation of femur on the tibia leading to false identifications of clinically undesirable contact kinematics (e.g., internal rotation and anterior translation during flexion). Our novel error correction algorithm is an effective method to account for this bias to more accurately compute contact kinematics.

Clinical outcome of increased flexion gap after total knee arthroplasty. Can controlled gap imbalance improve knee flexion?

PURPOSE

Increased range of motion (ROM) while maintaining joint stability is the goal of modern total knee arthroplasty (TKA). A biomechanical study has shown that small increases in flexion gap result in decreased tibiofemoral force beyond 90° flexion. The purpose of this paper was to investigate clinical implications of controlled increased flexion gap.

METHODS

Four hundred and four TKAs were allocated into one of two groups and analysed retrospectively. In the first group (n = 352), flexion gap exceeded extension gap by 2.5 mm, while in the second group (n = 52) flexion gap was equal to the extension gap. The procedures were performed from 2008 to 2012. The patients were reviewed 12 months postoperatively. Objective clinical results were assessed for ROM, mediolateral and sagittal stability. Patient-reported outcome measures were the WOMAC score and the Forgotten Joint Score (FJS-12).

RESULTS

After categorizing postoperative flexion into three groups (poor < 90°, satisfactory 91°-119°, good ≥ 120°) significantly more patients in group 1 achieved satisfactory or good ROM (p = 0.006). Group 1 also showed a significantly higher mean FJS-12 (group 1: 73, group 2: 61, p = 0.02). The mean WOMAC score was 11 in the first and 14 in the second group (n.s.). Increase in flexion gap did not influence knee stability.

CONCLUSIONS

The clinical relevance of this study is that a controlled flexion gap increase of 2.5 mm may have a positive effect on postoperative flexion and patient satisfaction after TKA. Neither knee stability in the coronal and sagittal planes nor complications were influenced by a controlled increase in flexion gap.

LEVEL OF EVIDENCE

III.

[Research progress of larger flexion gap than extension gap in total knee arthroplasty]

Objective

To summarize the progress of larger flexion gap than extension gap in total knee arthro-plasty (TKA).

Methods

The domestic and foreign related literature about larger flexion gap than extension gap in TKA, and its impact factors, biomechanical and kinematic features, and clinical results were summarized.

Results

During TKA, to adjust the relations of flexion gap and extension gap is one of the key factors of successful operation. The biomechanical, kinematic, and clinical researches show that properly larger flexion gap than extension gap can improve both the postoperative knee range of motion and the satisfaction of patients, but does not affect the stability of the knee joint. However, there are also contrary findings. So adjustment of flexion gap and extension gap during TKA is still in dispute.

Conclusion

Larger flexion gap than extension gap in TKA is a new joint space theory, and long-term clinical efficacy, operation skills, and related complications still need further study.

The biomechanical effect of increased valgus on total knee arthroplasty: a cadaveric study

The effects of valgus load on cadaveric knees following total knee arthroplasty (TKA) were investigated using a custom testing system. TKAs were performed on 8 cadaveric knees and tested at 0°, 30°, and 60° knee flexion in both neutral and 5° valgus. Fuji pressure sensitive film was used to quantify contact areas and pressures and MCL strain was determined using a Microscribe digitizing system. Lateral tibiofemoral pressures increased (P < 0.05) at all knee flexion angles with valgus loading. Patellofemoral contact characteristics did not change significantly (P > 0.05). Significant increases in strain were observed along the anterior and posterior border of the MCL at all knee flexion angles. These findings suggest that valgus loading increases TKA joint contact pressures and MCL strain with increasing knee flexion which may increase implant instability.

Differences in knee joint kinematics and forces after posterior cruciate retaining and stabilized total knee arthroplasty

BACKGROUND

Posterior cruciate ligament (PCL) retaining (CR) and -sacrificing (PS) total knee arthroplasties (TKA) are widely-used to treat osteoarthritis of the knee joint. The PS design substitutes the function of the PCL with a cam-spine mechanism which may produce adverse changes to joint kinematics and kinetics.

METHODS

CR- and PS-TKA were performed on 11 human knee specimens. Joint kinematics were measured with a dynamic knee simulator and motion tracking equipment. In-situ loads of the PCL and cam-spine were measured with a robotic force sensor system. Partial weight bearing flexions were simulated and external forces were applied.

RESULTS

The PS-TKA rotated significantly less throughout the whole flexion range compared to the CR-TKA. Femoral roll back was greater in the PS-TKA; however, this was not correlated with lower quadriceps forces. Application of external loads produced significantly different in-situ force profiles between the TKA systems.

CONCLUSIONS

Our data demonstrate that the PS-design significantly alters kinematics of the knee joint. Our data also suggest the cam-spine mechanism may have little influence on high flexion kinematics (such as femoral rollback) with most of the load burden shared by supporting implant and soft-tissue structures.

Three-dimensional motion analysis of the human knee joint: comparison between intra- and post-operative measurements

PURPOSE

To compare intra-operative knee joint kinematic measurements immediately after total knee replacement with those of the same patients post-operatively at 6-month follow-up.

METHODS

Fifteen patients who underwent total knee arthroplasty were analysed retrospectively. Eight were implanted with one prosthesis design and seven with another. The intra-operative measurements were performed by using a standard knee navigation system. This provided accurate three-dimensional positions and orientations for the femur and tibia by corresponding trackers pinned into the bones. At 6-month follow-up, the patients were analysed by standard three-dimensional video-fluoroscopy of the replaced knee during stair climbing, chair rising and step-up. Relevant three-dimensional positions and orientations were obtained by an iterative shape-matching procedure between the silhouette contours and the CAD-model projections. A number of traditional kinematic parameters were calculated from both measurements to represent the joint motion.

RESULTS

Good post-operative replication of the intra-operative measurements was observed for most of the variables analysed. The statistical analysis also supported the good consistency between the intra- and post-operative measurements.

CONCLUSIONS

Intra-operative kinematic measurements, accessible by a surgical navigation system, are predictive of the following motion performance of the replaced knees as experienced in typical activities of daily living.

LEVEL OF EVIDENCE

Prognostic studies--investigating natural history and evaluating the effect of a patient characteristic, Level II.

Mechanics of post-cam engagement during simulated dynamic activity

Posterior-stabilized (PS) total knee arthroplasty (TKA) components employ a tibial post and femoral cam mechanism to guide anteroposterior knee motion in lieu of the posterior cruciate ligament. Some PS TKA patients report a clicking sensation when the post and cam engage, while severe wear and fracture of the post; we hypothesize that these complications are associated with excessive impact velocity at engagement. We evaluated the effect of implant design on engagement dynamics of the post-cam mechanism and resulting polyethylene stresses during dynamic activity. In vitro simulation of a knee bend activity was performed for four cadaveric specimens implanted with PS TKA components. Post-cam engagement velocity and flexion angle at initial contact were determined. The experimental data were used to validate computational predictions of PS mechanics using the same loading conditions. A lower limb model was subsequently utilized to compare engagement mechanics of eight TKA designs, relating differences between implants to geometric design features. Flexion angle and post-cam velocity at engagement demonstrated considerable ranges among designs (23°-89°, and 0.05-0.22 mm/°, respectively). Post-cam velocity was correlated (r = 0.89) with tibiofemoral condylar design features. Condylar geometry, in addition to post-cam geometry, played a significant role in minimizing engagement velocity and forces and stresses in the post. This analysis guides selection and design of PS implants that facilitate smooth post-cam engagement and reduce edge loading of the post.

Intraoperative joint gaps affect postoperative range of motion in TKAs with posterior-stabilized prostheses

BACKGROUND

Joint gaps and mediolateral (ML) soft tissue balance have long been known to affect clinical scores and patient function after TKA, but the relationship between gaps and soft tissue balance remain poorly defined. If specific relationships exist between soft tissue tension and patient function, then objective targets could be established to assist surgeons in achieving more consistent postoperative knee function.

QUESTIONS/PURPOSES

By performing instrumented gap measurements during TKA, we sought to quantify the relationships between intraoperative soft tissue tension and clinical scores and patient function.

METHODS

We prospectively followed 57 patients with 63 primary TKAs with posterior-stabilized prostheses. Joint gaps and ML soft tissue balance were measured intraoperatively from 0° to 135° with the patella reduced after independent bone cuts and soft tissue releases. We determined the relationships between these intraoperative measurements and postoperative ROM and Knee Society scores at minimum 2-year followup.

RESULTS

Larger joint gaps at 120° and 135° flexion predicted larger postoperative knee flexion (r=0.296 and r=0.393, respectively), whereas larger gaps at 10° flexion predicted greater postoperative knee extension (r=0.285). Knees with rectangular joint gaps did not show better ROM or Knee Society scores compared with knees with trapezoidal joint gaps. In the range of normal surgical variation, neither joint gaps nor gap asymmetry affected the incidence of postoperative instability.

CONCLUSIONS

Avoiding small joint gaps in extension and in deep flexion should allow patients who undergo TKAs to obtain maximum ROM.

LEVEL OF EVIDENCE

Level II, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.

Sensitivity of knee soft-tissues to surgical technique in total knee arthroplasty

Restricted range of motion and excessive laxity are both potential complications of total knee arthroplasty (TKA). During TKA surgery, the surgeon is frequently faced with the question of how tightly to implant the prosthesis. The most common method of altering implantation tightness is to vary the thickness of the polyethylene inlay after the bone cuts have been made and the trial components inserted. We have sought to quantify how altering the polyethylene thickness may affect post-operative soft tissue tension for a range of prosthetic designs. Four different prosthetic designs were implanted into fresh-frozen cadaveric knee joints. All four designs were implanted in the standard manner, with a 100 Newton distraction force used to set soft tissue balance. The tibiofemoral force was then recorded at 15° intervals throughout the passive flexion range. After the standard implantation of each prosthesis, the tibial component was raised or lowered to mimic increasing and decreasing the polyethylene thickness by 2mm and the force measurements repeated. Tibiofemoral force in extension correlated with implantation tightness for all prosthesis designs. Between 15° and 90° of knee flexion, all four designs were insensitive to changes in implantation tightness. Beyond 90° the effect was more notable in rotating platform mobile-bearing and cruciate-retaining prostheses than in posterior-stabilised mobile-bearing designs. The findings of this research may be useful in assisting surgical decision-making during the implantation of TKA prostheses.

Comparison of two intraoperative assessment methods for injuries to the ankle syndesmosis. A cadaveric study

BACKGROUND

Intraoperative stress testing is required for the detection of syndesmosis instability following an ankle fracture. The present study compared two stress tests for the detection of syndesmotic injury.

METHODS

A true mortise radiograph of the ankle was made for fourteen cadaver joints. Specimens were randomized into two groups to simulate ligament and syndesmosis injury on the basis of the Danis-Weber classification system. In the first group, the anterior inferior tibiofibular ligament was divided first (Weber B(r)), followed sequentially by division of the interosseous membrane (Weber C) and the deltoid ligament. In the second group, the deltoid ligament was divided first, followed by the anterior inferior tibiofibular ligament. Radiographs were made at each stage with use of two methods of stressing the ankle mortise: (1) external rotation of the foot with an external moment of 7.5 Nm, and (2) application of a lateral force of 100 N. Tibiofibular overlap, tibiofibular clear space, and medial clear space were measured.

RESULTS

Lateral stress produced a significantly greater increase in the tibiofibular clear space than did the external rotation test for Weber C injuries and Weber C plus deltoid ligament injuries. A greater increase in the tibiofibular clear space was noted during the lateral stress test when both the deltoid and the anterior inferior tibiofibular ligament had been sectioned (p < 0.05). The external rotation stress test produced a significant increase in the medial clear space in the presence of isolated anterior inferior tibiofibular ligament and deltoid ligament injuries (p < 0.05).

CONCLUSIONS

For the detection of syndesmotic instability at the site of ankle fractures on stress radiographs, the lateral stress test appeared to be superior to the external rotation stress test in this cadaver model.

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 degrees and 135 degrees of flexion, and within 2 mm (+/-1.2 mm) throughout the entire range of motion (0-150 degrees). After insertion of a mobile-bearing TKA, the rod was displaced a mean of 6 mm at 150 degrees (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.

Length change patterns of the extensor retinaculum and the effect of total knee replacement

Patellofemoral dysfunction following total knee replacement (TKR) is a significant clinical problem, but little information exists on the mechanics of the patellofemoral retinacula or the effects of TKR on these structures. We hypothesized that TKR would cause significant elongation of the retinacula. Retinacular length changes were measured by threading sutures along the retinacula, fixing the sutures to the patella and the iliotibial band (ITB), and attaching the femoral ends to displacement transducers. The intact knee was flexed-extended while the quadriceps and ITB were tensed and the retinacular length change patterns were recorded. The measurements were repeated post-TKR. The medial patellofemoral ligament (MPFL) was close to isometric, stretching 2 mm in terminal knee extension, whereas the lateral retinaculum slackened 8 mm from 110 degrees to 0 degrees flexion. TKR did not cause significant elongation of either of the retinacula, the largest change being 3 mm elongation of the MPFL around 40 degrees , which stretched the MPFL by 1.4 mm above its maximum natural length. Thus, this work did not support the hypothesis that TKR causes significant elongation of the retinacula sufficient to affect knee function.

ESB Clinical Biomechanics Award 2008: Complete data of total knee replacement loading for level walking and stair climbing measured in vivo with a follow-up of 6-10 months

BACKGROUND

Detailed information about the loading of the knee joint is required for various investigations in total knee replacement. Up to now, gait analysis plus analytical musculo-skeletal models were used to calculate the forces and moments acting in the knee joint. Currently, all experimental and numerical pre-clinical tests rely on these indirect measurements which have limitations. The validation of these methods requires in vivo data; therefore, the purpose of this study was to provide in vivo loading data of the knee joint.

METHODS

A custom-made telemetric tibial tray was used to measure the three forces and three moments acting in the implant. This prosthesis was implanted into two subjects and measurements were obtained for a follow-up of 6 and 10 months, respectively. Subjects performed level walking and going up and down stairs using a self-selected comfortable speed. The subjects' activities were captured simultaneously with the load data on a digital video tape. Customized software enabled the display of all information in one video sequence.

FINDINGS

The highest mean values of the peak load components from the two subjects were as follows: during level walking the forces were 276%BW (percent body weight) in axial direction, 21%BW (medio-lateral), and 29%BW (antero-posterior). The moments were 1.8%BW*m in the sagittal plane, 4.3%BW*m (frontal plane) and 1.0%BW*m (transversal plane). During stair climbing the axial force increased to 306%BW, while the shear forces changed only slightly. The sagittal plane moment increased to 2.4%BW*m, while the frontal and transversal plane moments decreased slightly. Stair descending produced the highest forces of 352%BW (axial), 35%BW (medio-lateral), and 36%BW (antero-posterior). The sagittal and frontal plane moments increased to 2.8%BW*m and 4.6%BW*m, respectively, while the transversal plane moment changed only slightly.

INTERPRETATION

Using the data obtained, mechanical simulators can be programmed according to realistic load profiles. Furthermore, musculo-skeletal models can be validated, which until now often lacked the ability to predict properly the non-sagittal load values, e.g. varus-valgus and internal-external moments.

Relationship between joint gap difference and range of motion in total knee arthroplasty: a prospective randomised study between different platforms

The objective of this study was to investigate the range of motion (ROM) of the knee before and four years after total knee arthroplasty (TKA) with a mobile or fixed type of platform and to prospectively evaluate whether there was a difference in ligament balance between the platform types. The subjects were 68 patients involving 76 joints. The mobile type was used in 31 joints and fixed type in 45 joints by employing a prospective randomised method. The passive maximum ROM was measured using a goniometer before and four years after surgery. Also, the intraoperative knee ligament balance was measured. The postoperative extension ROM was significantly improved after TKA using a mobile bearing type compared with that employing a fixed bearing type. In TKA using the former, the intraoperative gap difference was not related to the postoperative flexion angle of the knee. However, they were related in TKA using a fixed bearing type, with a positive correlation regarding the flexion gap.

High-flexion total knee arthroplasty

High-flexion total knee arthroplasty is considered flexion beyond 125 degrees . Certain activities and a number of workplace demands benefit from this greater range of motion. Some cultures and religions place more emphasis on deep knee flexion. Important patient factors include preoperative motion, body mass index, and previous knee surgery. Component design modifications focus on lengthening the radius of curvature through the posterior condyles, increasing the posterior condylar offset, recessing the tibial insert, lengthening the trochlear groove, and altering the cam-post design. These changes allow increased femoral rollback, translation, and thus clearance in deep flexion. Surgical techniques focus on soft tissue balancing, component sizing and position, removal of impinging osteophytes, and reestablishment of the flexion gap. A number of outcome studies have demonstrated benefits for high flexion after standard total knee and high-flexion designs.

In vivo contact kinematics and contact forces of the knee after total knee arthroplasty during dynamic weight-bearing activities

Analysis of polyethylene component wear and implant loosening in total knee arthroplasty (TKA) requires precise knowledge of in vivo articular motion and loading conditions. This study presents a simultaneous in vivo measurement of tibiofemoral articular contact forces and contact kinematics in three TKA patients. These measurements were accomplished via a dual fluoroscopic imaging system and instrumented tibial implants, during dynamic single leg lunge and chair rising-sitting. The measured forces and contact locations were also used to determine mediolateral distribution of axial contact forces. Contact kinematics data showed a medial pivot during flexion of the knee, for all patients in the study. Average axial forces were higher for lunge compared to chair rising-sitting (224% vs. 187% body weight). In this study, we measured peak anteroposterior and mediolateral forces averaging 13.3% BW during lunge and 18.5% BW during chair rising-sitting. Mediolateral distributions of axial contact force were both patient and activity specific. All patients showed equitable medial-lateral loading during lunge but greater loads at the lateral compartment during chair rising-sitting. The results of this study may enable more accurate reproduction of in vivo loads and articular motion patterns in wear simulators and finite element models. This in turn may help advance our understanding of factors limiting longevity of TKA implants, such as aseptic loosening and polyethylene component wear, and enable improved TKA designs.