Femoral and tibial insert downsizing increases the laxity envelope in TKA

Biomechanical Consequences of Tibial Insert Thickness after Total Knee Arthroplasty: A Musculoskeletal Simulation Study

The thickness of the tibial polyethylene (PE) insert is a critical parameter to ensure optimal soft-tissue balancing in the intraoperative decision-making procedure of total knee arthroplasty (TKA). However, there is a paucity of information about the kinetic response to PE insert thickness variations in the tibiofemoral (TF) joint, and subsequently, the secondary effects on the patellofemoral (PF) biomechanics. Therefore, the purpose of this study was to investigate the influence of varying PE insert thickness on the ligament and TF compressive forces, as well as on the PF forces and kinematics, after a cruciate-retaining TKA. A previous patient-specific musculoskeletal model of TKA was adapted to simulate a chair-rising motion in which PE insert thickness was varied with 2 mm increments or decrements compared to the reference case (9 mm), from 5 mm up to 13 mm. Greater PE insert thickness resulted in higher ligament forces and concurrently increased the TF compressive force by 21% (13 mm), but slightly unloaded the PF joint with 7% (13 mm) while shifting the patella distally in the trochlear groove, compared to the reference case. Thinner PE inserts showed an opposite trend. Our findings suggest that the optimal PE insert thickness selection is a trade-off between the kinetic outcomes of the TF and PF joints.

Influence of Narrow Femoral Implants on Intraoperative Soft Tissue Balance in Posterior-Stabilized Total Knee Arthroplasty

BACKGROUND

Narrow femoral implants were developed to improve fit and prevent overhang in primary total knee arthroplasty (TKA). We compared intraoperative soft tissue balance between standard and narrow implants in posterior-stabilized (PS) TKA.

METHODS

We enrolled 30 consecutive patients with varus osteoarthritis undergoing PS TKA using an image-free navigation system. Standard and narrow femoral trial implants were inserted, and their soft tissue balance was measured. Subgroup analysis, based on the actual implanted femoral implant, was performed to assess the influence of narrow implants on soft tissue balance.

RESULTS

Narrow trial group had significantly larger joint component gaps than standard trial group at all measured flexion angles, except at 60° (P < .05). For the standard implant cohort, narrow trial group had significantly larger joint component gaps than standard trial group at 30°, 120°, and 135° flexion (P < .05). For the narrow implant cohort, narrow trial group had significantly larger joint component gaps than standard trial group at all measured flexion angles, except at 0° and 60° (P < .05). Narrow trial group had significantly larger varus ligament balance than standard trial group at 45° and 60° flexion (P < .05). The varus angles for standard implants were comparable between groups; however, narrow trial group had significantly larger varus angles for narrow implants than standard trial group at 45°, 60°, and 120° flexion (P < .05).

CONCLUSION

The medial-lateral dimension and volume of the femoral component may influence intraoperative soft tissue balance in PS TKA. The effects may be greater when narrow implants are selected to avoid component overhang.

What Is the Possible Impact of High Variability of Distal Femoral Geometry on TKA? A CT Data Analysis of 24,042 Knees

BACKGROUND

Previous studies analyzing femoral components of TKAs have demonstrated the limited ability of these components to accommodate size variations seen in the patient population, particularly width and femoral offset.

QUESTIONS/PURPOSES

The purpose of this study was to use a large data set of knee CT scans (1) to determine the variations in the distal and posterior femoral geometries and to determine whether there is a correlation between distal condylar offset and posterior femoral offset as a potential parameter for symmetry/asymmetry; and (2) to evaluate what proportion of knees would have a substantial mismatch between the implant's size or shape and the patient's anatomy if a femoral component of a modern standard TKA of symmetric (sTKA) or asymmetric (asTKA) designs were to be used.

METHODS

A retrospective study was performed on 24,042 data sets that were generated during the design phase for a customized TKA implant. This data set was drawn from European and US-American patients. Measurements recorded for the femur included the overall AP and mediolateral (ML) widths, widths of the lateral condyle and the medial condyle, the distal condylar offset (DCO) between the lateral and medial condyles in the superoinferior direction, and the posterior femoral offset (PFO) as the difference between the medial and lateral posterior condylar offset (PCO) measured in the AP direction. A consecutively collected subset of 2367 data sets was further evaluated to determine the difference between the individual AP and ML dimensions of the femur with that of modern TKA designs using two commercially available implants from different vendors.

RESULTS

We observed a high degree of variability in AP and ML widths as well as in DCO and PFO. Also, we found no correlation between DCO and PCO of the knees studied. Instances of a patient having a small DCO and higher PCO were commonly seen. Analysis of the DFOs revealed that overall, 62% (14,906 of 24,042) of knees exhibited DCO > 1 mm and 83% (19,955 of 24,042) of femurs exhibited a > 2-mm difference between the lateral and medial PCO. Concerning AP and ML measurements, 23% (544 of 2367) and 25% (592 of 2367) would have a mismatch between the patient's bony anatomy and the dimensions of the femoral component of ± 3 mm if they would have undergone a modern standard sTKA or asTKA design, respectively.

CONCLUSIONS

Analysis of a large number of CT scans of the knee showed that a high degree of variability exists in AP and ML widths as well as in DCO and PFO.

CLINICAL RELEVANCE

These findings suggest that it is possible that a greater degree of customization could result in surgeons performing fewer soft tissue releases and medial resections than now are being done to fit a fixed-geometry implant into a highly variable patient population. However, as an imaging study, it cannot support one approach to TKA over another; comparative studies that assess patient-reported outcomes and survivorship will be needed to help surgeons decide among sTKA, asTKA, and customized TKA.

Analysis of differences in laxities and neutral positions from native after kinematically aligned TKA using cruciate retaining implants

One biomechanical goal of kinematically aligned total knee arthroplasty (KA TKA) is to achieve knee laxities and neutral positions that are not different from those of the native knee without soft tissue release. However, replacing the articular surfaces and menisci with implants of discrete sizes and average shapes and resecting the anterior cruciate ligament (ACL) might prevent KA TKA from achieving this goal in the tibiofemoral joint. Accordingly, the objective was to determine whether either or both surgically induced changes cause differences in laxities and/or neutral positions from native using a cruciate retaining implant. Eight laxities and four neutral positions were measured from 0° to 120° flexion in 30° increments in 13 human cadaveric knees in three knee conditions: native, ACL-deficient, and KA TKA. After KA TKA, 5 of the 40 laxity measures (8 laxities × 5 flexion angles) and 6 of the 20 neutral position measures (4 neutral positions × 5 flexion angles) were statistically different from those of the native knee. The greatest differences in laxities from native after KA TKA occurred at 30° flexion in anterior translation (1.6 ± 2.1 mm increase, p < 0.0001); this difference was 1.7 ± 2.1 mm less than that in the ACL-d knee (p < 0.0001). The greatest difference in neutral positions from native after KA TKA occurred in anterior-posterior translation at 0° flexion (3.8 ± 1.9 mm anterior, p < 0.0001); this difference was 2.6 ± 1.9 mm greater than that in the ACL-d knee (p = 0.0002). Clinical Significance: These results indicate that the biomechanical goal of KA TKA is largely realized despite the two surgically induced changes. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:358-369, 2019.

Increases in tibial force imbalance but not changes in tibiofemoral laxities are caused by varus-valgus malalignment of the femoral component in kinematically aligned TKA

PURPOSE

The purposes of this study were to quantify the increase in tibial force imbalance (i.e. magnitude of difference between medial and lateral tibial forces) and changes in laxities caused by 2° and 4° of varus-valgus (V-V) malalignment of the femoral component in kinematically aligned total knee arthroplasty (TKA) and use the results to detemine sensitivities to errors in making the distal femoral resections. Because V-V malalignment would introduce the greatest changes in the alignment of the articular surfaces at 0° flexion, the hypotheses were that the greatest increases in tibial force imbalance would occur at 0° flexion, that primarily V-V laxity would significantly change at this flexion angle, and that the tibial force imbalance would increase and laxities would change in proportion to the degree of V-V malalignment.

METHODS

Kinematically aligned TKA was performed on ten human cadaveric knee specimens using disposable manual instruments without soft tissue release. One 3D-printed reference femoral component, with unmodified geometry, was aligned to restore the native distal and posterior femoral joint lines. Four 3D-printed femoral components, with modified geometry, introduced V-V malalignments of 2° and 4° from the reference component. Medial and lateral tibial forces were measured during passive knee flexion-extension between 0° to 120° using a custom tibial force sensor. Eight laxities were measured from 0° to 120° flexion using a six degree-of-freedom load application system.

RESULTS

With the tibial component kinematically aligned, the increase in the tibial force imbalance from that of the reference component at 0° of flexion was sensitive to the degree of V-V malalignment of the femoral component. Sensitivities were 54 N/deg (medial tibial force increasing > lateral tibial force) (p < 0.0024) and 44 N/deg (lateral tibial force increasing > medial tibial force) (p < 0.0077) for varus and valgus malalignments, respectively. Varus-valgus malalignment did not significantly change varus, internal-external rotation, anterior-posterior, and compression-distraction laxities from 0° to 120° flexion. At only 30° of flexion, 4° of varus malalignment increased valgus laxity 1° (p = 0.0014).

CONCLUSION

At 0° flexion, V-V malalignment of the femoral component caused the tibial force imbalance to increase significantly, whereas the laxities were relatively unaffected. Because tibial force imbalance has the potential to adversely affect patient-reported outcomes and satisfaction, surgeons should strive to limit errors in resecting the distal femoral condyles to within ± 0.5 mm which in turn limits the average increase in tibial force imbalance to 68 N. Because laxities were generally unaffected, instability resulting from large increases in laxity is not a clinical concern within the ± 4° range tested.

LEVEL OF EVIDENCE

Therapeutic, Level II.

Posterior Reference Position Affects Intraoperative Kinematic and Soft Tissue Balance in Navigated Posterior-Stabilized Total Knee Arthroplasty

BACKGROUND

The importance of medial compartment stability is recognized in total knee arthroplasty (TKA). To manage the medial extension-flexion gap, the posterior reference position can be changed from conventional posterior center to posterior medial in measured resection techniques. This study aimed to compare the intraoperative soft tissue balance and rotational kinematics between the posterior medial and posterior center reference groups.

METHODS

We enrolled 57 consecutive patients with varus osteoarthritis undergoing posterior-stabilized (PS) TKA using an image-free navigation system. The detailed surgical plan in both groups and intraoperative kinematics were recorded using navigation, and soft tissue balance measured with an offset-type tensor was statistically compared between groups.

RESULTS

Patients were divided into the posterior center reference group (n = 32) and posterior medial reference group (n = 25). The posterior medial and posterior lateral condyles were significantly thicker in the posterior center reference group (P < .05). Although preoperative rotational kinematics were comparable between groups, the tibial rotational position was significantly more externally rotated in the posterior center reference group than in the posterior medial reference group at 45°, 60°, and 90° of flexion (P < .05). The varus angle and joint component gap were significantly smaller in the posterior medial reference group than in the posterior center reference group at 60° and 90° of flexion (P < .05).

CONCLUSION

The posterior reference position affects intraoperative kinematics and soft tissue balance in navigated PS TKA. Posterior medial reference PS TKA decreases the excessive tibial external rotation during midflexion and increases the flexion stability compared with conventional posterior center reference PS TKA.

Tibial shaft anatomy differs between Caucasians and East Asian individuals

PURPOSE

The orientation and distance from the shaft of the femur and tibia to the articular surface centre is important for performing total knee arthroplasty (TKA) with cementless stems. It is important to understand anatomic differences between races to match the tibial and femoral shaft axis to the knee articular surface. Thus, the purpose of this study was to compare knee morphology between Caucasian and East Asian individuals to determine the optimal placement of tibial and femoral stems.

METHODS

A retrospective study was conducted on a matched cohort of 50 East Asians (21F, 29M) and 50 Caucasians (21F, 29M) by age and gender. CT scans were obtained in healthy volunteers using < 2-mm slices. The distance from the proximal tibial diaphysis axis to the tibial plateau centre and the distance from the distal femoral diaphysis axis to the centre of distal femoral articular surface were measured separately. Tibial measurements were taken using Akagi's anteroposterior (AP) axis and the widest mediolateral (ML) diameter, and femoral measurements were based on Whiteside's line and the surgical epicondylar axis.

RESULTS

The ML distance between the tibial shaft centre and Akagi's line was significantly higher for Asians (9.9 ± 2.7 mm, Caucasians 7.7 ± 3.1 mm, p < 0.001). The distance between the femoral shaft centre and Whiteside's line was lower, although not significantly different (Asians 1.9 ± 1.0 mm, Caucasians 2.2 ± 1.1 mm, n.s.). However, there were no differences in the AP dimension for the femur or tibia comparing Asians to Caucasians in both tibial side (Asians 10.6 ± 3.3 mm vs. Caucasians 10.9 ± 4.0 mm) and femoral side (Asians 18.1 ± 1.7 mm vs. Caucasians 17.5 ± 1.6 mm).

CONCLUSION

East Asian individuals have more offset in the ML dimension for the tibia. This finding is clinical relevant, as this dimensional difference should be taken into consideration when designing primary and revision TKA stemmed tibial implants for East Asian individuals.

LEVEL OF EVIDENCE

Case-control study, Level III.

Internal-external malalignment of the femoral component in kinematically aligned total knee arthroplasty increases tibial force imbalance but does not change laxities of the tibiofemoral joint

PURPOSE

The purposes of this study were to quantify the increase in tibial force imbalance (i.e. magnitude of difference between medial and lateral tibial forces) and changes in laxities caused by  2° and 4° of internal-external (I-E) malalignment of the femoral component in kinematically aligned total knee arthroplasty. Because I-E malalignment would introduce the greatest changes to the articular surfaces near 90° of flexion, the hypotheses were that the tibial force imbalance would be significantly increased near 90° flexion and that primarily varus-valgus laxity would be affected near 90° flexion.

METHODS

Kinematically aligned TKA was performed on ten human cadaveric knee specimens using disposable manual instruments without soft tissue release. One 3D-printed reference femoral component, with unmodified geometry, was aligned to restore the native distal and posterior femoral joint lines. Four 3D-printed femoral components, with modified geometry, introduced I-E malalignments of 2° and 4° from the reference component. Medial and lateral tibial forces were measured from 0° to 120° flexion using a custom tibial force sensor. Bidirectional laxities in four degrees of freedom were measured from 0° to 120° flexion using a custom load application system.

RESULTS

Tibial force imbalance increased the greatest at 60° flexion where a regression analysis against the degree of I-E malalignment yielded sensitivities (i.e. slopes) of 30 N/° (medial tibial force > lateral tibial force) and 10 N/° (lateral tibial force > medial tibial force) for internal and external malalignments, respectively. Valgus laxity increased significantly with the 4° external component with the greatest increase of 1.5° occurring at 90° flexion (p < 0.0001).

CONCLUSION

With the tibial component correctly aligned, I-E malalignment of the femoral component caused significant increases in tibial force imbalance. Minimizing I-E malalignment lowers the increase in the tibial force imbalance. By keeping the resection thickness of each posterior femoral condyle to within ± 0.5 mm of the thickness of the respective posterior region of the femoral component, the increase in imbalance can be effectively limited to 38 N. Generally laxities were unaffected within the ± 4º range tested indicating that instability is not a clinical concern and that manual testing of laxities is not useful to detect I-E malalignment.

Association between anteroposterior laxity in mid-range flexion and subjective healing of instability after total knee arthroplasty

PURPOSE

Flexion instability following total knee arthroplasty (TKA) is a common indication of early revision. The association between the objective anteroposterior (AP) laxity direction in mid-range flexion and the subjective healing of instability remains unclear; thus, this study aimed to clarify this association.

METHODS

In this study, 110 knees (74 females, 92 knees; 16 males, 18 knees) with medial pivot implants were examined with a median age of 79 (range 60-92) years for a median follow-up duration of 22 (range 6-125) months. AP laxity was measured using a KT-1000 arthrometer. Self-reported knee instability score was used for the subjective healing of instability.

RESULTS

Eighty-seven knees did not feel unstable (Group 0), whereas 23 knees felt unstable (Group 1). There was a significant difference in AP displacement [Group 0: median 6 mm; range 2-15 mm and Group 1: median 8 mm; range 4-14; p < 0.0001]. The threshold value of 7 mm was determined using the area under receiver operating characteristic curve of 0.79 [95% confidence interval (CI) 0.69-0.88, p < 0.0001]. In multivariate analysis, AP displacement of ≥7 mm was an independent risk factor for feelings of instability (odds ratio 7.695; 95% CI 2.306-25.674; p = 0.001).

CONCLUSIONS

AP laxity of ≥7 mm represents a known cause of feelings of instability. By controlling AP laxity in TKAs, without stiffness in the knee, it is possible to prevent feelings of instability. The clinical relevance is that AP laxity of <7 mm is one of the target areas in TKA.

LEVEL OF EVIDENCE

IV.

Effect of Femoral Component Flexion Implantation on the Mediolateral Bone-prosthetic Fit in Total Knee Arthroplasty

OBJECTIVE

Femoral component overhang in total knee arthroplasty (TKA) has been reported in previous studies. The purpose of this study was to evaluate the effect of femoral component flexion implantation on mediolateral bone-prosthetic fit in TKA.

METHODS

Virtual prosthesis implantations were performed on computed tomographic models of 10 Chinese knees with femoral prostheses of the Advance Medial-Pivot knee system (MicroPort Orthopedics, Arlington, TN, USA), with the femoral component positioned at 0°, 3°, or 6° of flexion in the sagittal plane. For each degree of flexion implantation, the differences between the knee and femoral component models on the lateral and medial sides at trochlea (zone 1), anterior-distal condyle (zone 2), posterior-distal condyle (zone 3), and posterior condyle (zone 4) were measured. Positive difference values indicate component overhang, and negative difference values indicate component underhang. The values of component overhang (underhang) in each zone were statistically analyzed across the 3° of flexion implantation.

RESULTS

With a greater degree of flexion implantation, overhang was reduced and even changed to underhang. With 0° of flexion implantation, an overhang exceeding 3 mm existed mainly on the medial side of zone 1 (5.81 mm) and the lateral side of zone 2 (3.39 mm). With 3° of flexion, overhang exceeding 3 mm was observed only on the medial side of zone 1 (3.10 mm), and underhang was observed only on the medial side of zone 4 (-0.32 mm). No overhang exceeding 3 mm was observed for 6° of flexion, while underhang was observed except on the lateral sides of zone 2 (1.32 mm) and zone 4 (1.10 mm) and on the medial side of zone 1 (1.54 mm). A significant difference in overhang values on the lateral and medial sides of zone 1 was observed between 0 and 6° of flexion (P < 0.05).

CONCLUSION

The present study demonstrated that femoral component flexion implantation by 3° can reduce excessive overhang, although 3.10 mm of overhang remained at the medial side of zone 1. Conversely, 6° of flexion implantation can avoid 3 mm of overhang for any zone, but increases the risk of underhang. Slight flexion implantation may be an effective alternative technique to prevent excessive component overhang, especially in the trochlea and anterior region of the distal condyle, in Chinese patients with standard TKA prostheses.

Morphological evaluation of the sagittal plane femoral load-bearing surface in computer-simulated virtual total knee arthroplasty implantation at different flexion angles

PURPOSE

To examine the effect of implantation of the femoral component of a total knee arthroplasty (TKA) system in 0°, 3°, and 6° of flexion on the sagittal plane morphology of the femoral load-bearing surfaces. It was hypothesized that increasing the flexion angle would result in undersizing of the anterior surface without changing the flexion gap.

METHODS

Computer simulation of a TKA using three-dimensional models of 10 healthy knees, matched to three different sized femoral components. Size discrepancy in the sagittal plane anterior, distal, and posterior joint surfaces between the native and prosthetic knees was calculated at 0°, 3°, and 6° of flexion.

RESULTS

The required component size varied with the angle of implantation: 0°, size 3/size 4 (N = 7/3), 3°, size 3 (N = 10); and 6°, size 2/size 3 (N = 4/6). Component undersizing ranged between 4.4-6.3 mm at the anterior lateral surface, with a significant difference between 0° and 6° (p < 0.05), and 1.2-3.5 mm at the anterior medial surface. Component oversizing of the distal surface of the lateral condyle (2.9 mm) and undersizing of the medial surface of the posterior condyle (1.6-2.3 mm) were comparable at all three flexion angles of component implantation.

CONCLUSIONS

Increasing the flexion angle of implantation increased the incidence of using a smaller size of femoral component without significant interference with the flexion gap. However, the effect of a smaller femoral component on undersizing of the anterior surface of the condyle and the impact on the extensor mechanism need to be considered.

A novel physiological testing device to study knee biomechanics in vitro

BACKGROUND

To properly study knee kinetics, kinematics and the effects of injury and surgical treatment in vitro, the knee should be constrained as little as possible, while imposing physiological loads. A novel dynamic biomechanical knee system (BKS) is presented here. The aim of this study was to test the feasibility and reproducibility of the system and demonstrate its features with an Anterior Cruciate Ligament (ACL) lesion model.

METHODS

Six goat knees were used in the current study. Flexion and extension simulating gait was imposed by a servo-motor, while normal joint load was applied by two artificial muscles. Intra-class correlation coefficients (ICCs) were assessed for inter-test measures, while paired t-tests were performed for comparison between intact knees and knees with ACL-lesion.

RESULTS

The ICC's for inter-test measures based on all six goat knees were excellent: varus/valgus: ICC=0.93; rotation: ICC=0.94 (all p<0.01), and translation in frontal (x)-, side (y)- and upward (z)-direction (ICC=0.90, 0.88 & 0.94) (all p<0.01). A significant increase in joint center movement was found in knees after creating an ACL-lesion (p=0.018): translation increased more than two-fold in frontal (p=0.016), side (p=0.004) and upward (p=0.018) direction.

CONCLUSIONS

Five degrees of motion were reproducibly assessed in the intact joint, suggesting that the goat knee may find its natural pathway when loaded in the BKS. The novel five-degrees-of-freedom knee system allows a detailed study of the effect of a diversity of defects and surgical treatments on knee biomechanics under physiological loading conditions.

An in vitro analysis of medial structures and a medial soft tissue reconstruction in a constrained condylar total knee arthroplasty

PURPOSE

The aim of this study was to quantify the medial soft tissue contributions to stability following constrained condylar (CC) total knee arthroplasty (TKA) and determine whether a medial reconstruction could restore stability to a soft tissue-deficient, CC-TKA knee.

METHODS

Eight cadaveric knees were mounted in a robotic system and tested at 0°, 30°, 60°, and 90° of flexion with ±50 N anterior-posterior force, ±8 Nm varus-valgus, and ±5 Nm internal-external torque. The deep and superficial medial collateral ligaments (dMCL, sMCL) and posteromedial capsule (PMC) were transected and their relative contributions to stabilising the applied loads were quantified. After complete medial soft tissue transection, a reconstruction using a semitendinosus tendon graft was performed, and the effect on kinematic behaviour under equivocal conditions was measured.

RESULTS

In the CC-TKA knee, the sMCL was the major medial restraint in anterior drawer, internal-external, and valgus rotation. No significant differences were found between the rotational laxities of the reconstructed knee to the pre-deficient state for the arc of motion examined. The relative contribution of the reconstruction was higher in valgus rotation at 60° than the sMCL; otherwise, the contribution of the reconstruction was similar to that of the sMCL.

CONCLUSION

There is contention whether a CC-TKA can function with medial deficiency or more constraint is required. This work has shown that a CC-TKA may not provide enough stability with an absent sMCL. However, in such cases, combining the CC-TKA with a medial soft tissue reconstruction may be considered as an alternative to a hinged implant.

Current state of computer navigation and robotics in unicompartmental and total knee arthroplasty: a systematic review with meta-analysis

Recently, there is a growing interest in surgical variables that are intraoperatively controlled by orthopaedic surgeons, including lower leg alignment, component positioning and soft tissues balancing. Since more tight control over these factors is associated with improved outcomes of unicompartmental knee arthroplasty and total knee arthroplasty (TKA), several computer navigation and robotic-assisted systems have been developed. Although mechanical axis accuracy and component positioning have been shown to improve with computer navigation, no superiority in functional outcomes has yet been shown. This could be explained by the fact that many differences exist between the number and type of surgical variables these systems control. Most systems control lower leg alignment and component positioning, while some in addition control soft tissue balancing. Finally, robotic-assisted systems have the additional advantage of improving surgical precision. A systematic search in PubMed, Embase and Cochrane Library resulted in 40 comparative studies and three registries on computer navigation reporting outcomes of 474,197 patients, and 21 basic science and clinical studies on robotic-assisted knee arthroplasty. Twenty-eight of these comparative computer navigation studies reported Knee Society Total scores in 3504 patients. Stratifying by type of surgical variables, no significant differences were noted in outcomes between surgery with computer-navigated TKA controlling for alignment and component positioning versus conventional TKA (p = 0.63). However, significantly better outcomes were noted following computer-navigated TKA that also controlled for soft tissue balancing versus conventional TKA (mean difference 4.84, 95 % Confidence Interval 1.61, 8.07, p = 0.003). A literature review of robotic systems showed that these systems can, similarly to computer navigation, reliably improve lower leg alignment, component positioning and soft tissues balancing. Furthermore, two studies comparing robotic-assisted with computer-navigated surgery reported superiority of robotic-assisted surgery in controlling these factors. Manually controlling all these surgical variables can be difficult for the orthopaedic surgeon. Findings in this study suggest that computer navigation or robotic assistance may help managing these multiple variables and could improve outcomes. Future studies assessing the role of soft tissue balancing in knee arthroplasty and long-term follow-up studies assessing the role of computer-navigated and robotic-assisted knee arthroplasty are needed.

The superficial medial collateral ligament is the primary medial restraint to knee laxity after cruciate-retaining or posterior-stabilised total knee arthroplasty: effects of implant type and partial release

PURPOSE

The aim of this study was to quantify the contributions of medial soft tissues to stability following cruciate-retaining (CR) or posterior-stabilised (PS) total knee arthroplasty (TKA).

METHODS

Using a robotic system, eight cadaveric knees were subjected to ±90-N anterior-posterior force, ±5-Nm internal-external and ±8-Nm varus-valgus torques at various flexion angles. The knees were tested intact and then with CR and PS implants, and successive cuts of the deep and superficial medial collateral ligaments (dMCL, sMCL) and posteromedial capsule (PMC) quantified the percentage contributions of each structure to restraining the applied loads.

RESULTS

In implanted knees, the sMCL restrained valgus rotation (62 % across flexion angles), anterior-posterior drawer (24 and 10 %, respectively) and internal-external rotation (22 and 37 %). Changing from CR TKA to PS TKA increased the load on the sMCL when resisting valgus loads. The dMCL restrained 11 % of external and 13 % of valgus rotations, and the PMC was significant at low flexion angles.

CONCLUSIONS

This work has shown that medial release in the varus knee should be minimised, as it may inadvertently result in a combined laxity pattern. There is increasing interest in preserving constitutional varus in TKA, and this work argues for preservation of the sMCL to afford the surgeon consistent restraint and maintain a balanced knee for the patient.

Morphometric analysis of the distal femur in total knee arthroplasty and native knees

AIMS

Analysis of the morphology of the distal femur, and by extension of the femoral components in total knee arthroplasty (TKA), has largely been related to the aspect ratio, which represents the width of the femur. Little is known about variations in trapezoidicity (i.e. whether the femur is more rectangular or more trapezoidal). This study aimed to quantify additional morphological characteristics of the distal femur and identify anatomical features associated with higher risks of over- or under-sizing of components in TKA.

METHODS

We analysed the shape of 114 arthritic knees at the time of primary TKA using the pre-operative CT scans. The aspect ratio and trapezoidicity ratio were quantified, and the post-operative prosthetic overhang was calculated. We compared the morphological characteristics with those of 12 TKA models.

RESULTS

There was significant variation in both the aspect ratio and trapezoidicity ratio between individuals. Femoral trapezoidicity was mostly due to an inward curve of the medial cortex. Overhang was correlated with the aspect ratio (with a greater chance of overhang in narrow femurs), trapezoidicity ratio (with a greater chance in trapezoidal femurs), and the tibio-femoral angle (with a greater chance in valgus knees).

DISCUSSION

This study shows that rectangular/trapezoidal variability of the distal femur cannot be ignored. Most of the femoral components which were tested appeared to be excessively rectangular when compared with the bony contours of the distal femur. These findings suggest that the design of TKA should be more concerned with matching the trapezoidal/rectangular shape of the native femur.

TAKE HOME MESSAGE

The distal femur is considerably more trapezoidal than most femoral components, and therefore, care must be taken to avoid anterior prosthetic overhang in TKA

Level of constraint in revision knee arthroplasty

Revision total knee arthroplasty (TKA) in the setting of major bone deficiency and/or soft tissue laxity might require increasing levels of constraint to restore knee stability. However, increasing the level of constraint not always correlates with mid-to-long-term satisfactory results. Recently, modular components as tantalum cones and titanium sleeves have been introduced to the market with the goal of obtaining better fixation where bone deficiency is an issue; theoretically, satisfactory meta-diaphyseal fixation can reduce the mechanical stress at the level of the joint line, reducing the need for high levels of constraint. This article reviews the recent literature on the surgical management of the unstable TKA with the goal to propose a modern surgical algorithm for adult reconstruction surgeons.

The limits of passive motion are variable between and unrelated within normal tibiofemoral joints

Patient-to-patient differences should be accounted for in both clinical evaluations and computational models of knee laxity. Accordingly, the objectives were to determine how variable the laxities are between knees by determining the range of the internal-external (I-E), varus-valgus (V-V), anterior-posterior (A-P), and compression-distraction (C-D) limits of passive motion, and how related the laxities are within a knee by determining whether these limits are correlated with one another. The limits in I-E (± 3 Nm), V-V (± 5 Nm), A-P (± 45 N), and C-D (± 100 N) were measured in 10 normal human cadaveric knees at 0° to 120° flexion in 15° increments using a six degree-of-freedom load application system. The ranges from 15° to 120° flexion of the I-E limits were greater than 3.6°, of the A-P limits were greater than 1.8 mm, and of the varus limits were greater than 1.4°. The ranges from 30° to 120° flexion of the distraction limits were greater than 2.0 mm. Twenty-four of the 28 pair-wise comparisons between the limits had a correlation coefficient less than 0.65. These results demonstrate that a patient-specific approach, including all degrees of freedom of interest, is necessary during clinical evaluations of laxity and when creating and validating computational models of the tibiofemoral joint.