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

Finite element analysis of femoral component sagittal alignment in mobile-bearing total knee arthroplasty

BACKGROUND: Recently, there has been an increasing interest in mobile-bearing total knee arthroplasty (TKA). However, changes in biomechanics for femoral component alignment in mobile-bearing TKA have not been explored in depth. OBJECTIVE: This study aims to evaluate the biomechanical effect of sagittal alignment of the femoral component in mobile-bearing TKA. METHODS: We developed femoral sagittal alignment models with −3°, 0°, 3°, 5°, and 7° flexion. We also examine the kinematics of the tibiofemoral (TF) joint, contact point on the TF joint, contact stress on the patellofemoral (PF) joint, collateral ligament force, and quadriceps force using a validated computational model under a deep-knee-bend condition. RESULTS: Posterior kinematics of the TF joint increases as the femoral component flexes. The contact stress on the PF joint, collateral ligament force, and the quadriceps force decreases as the femoral component flexes. CONCLUSIONS: Our results show that a slight, approximately 0°∼3°, flexion of the implantation could be an effective substitute technique. However, excessive flexion should be avoided because of the potential loosening of the TF joint.

In Vivo Knee Kinematics for a Cruciate Sacrificing Total Knee Arthroplasty Having Both a Symmetrical Femoral and Tibial Component

BACKGROUND

Early total knee arthroplasty (TKA) designs were symmetrical, but lead to complications due to over-constraint leading to loosening and poor flexion. Next-generation TKAs have been designed to include asymmetry, pertaining to the trochlear groove, femoral condylar shapes, and/or the tibial component. More recently, an advanced posterior cruciate sacrificing (PCS) TKA was designed to include both a symmetrical femoral component with a patented V-shaped trochlear groove and a symmetrical tibial component with an ultracongruent insert, in an attempt to reduce inventory costs. Because previous PCS TKA designs produced variable results, the objective of this study is to determine and evaluate the in vivo kinematics for subjects implanted with this symmetrical TKA.

METHODS

Twenty-one subjects, implanted with symmetrical PCS fixed-bearing TKA, were asked to perform a weight-bearing deep knee bend (DKB) while under fluoroscopic surveillance. A 3-dimensional to 2-dimensional registration technique was used to determine each subject's anteroposterior translation of lateral and medial femoral condyles as well as tibiofemoral axial rotation and their weight-bearing knee flexion.

RESULTS

During the DKB, the average active maximum weight-bearing flexion was 111.7° ± 13.3°. On average, from full extension to maximum knee flexion, subjects experienced -2.5 ± 2.0 mm of posterior femoral rollback of the lateral condyle and 2.5 ± 2.2 mm of medial condyle motion in the anterior direction. This medial condyle motion was consistent for the majority of the subjects, with the lateral condyle exhibiting rollback from 0° to 60° of flexion and then experienced an average anterior motion of 0.3 mm from 60° to 90° of knee flexion. On average, the subjects in this study experienced 6.6°± 3.3° of axial rotation, with most of the rotation occurring in early flexion, averaging 4.9°.

CONCLUSION

Although subjects in this study were implanted with a symmetrical PCS TKA, they did experience femoral rollback of the lateral condyle and a normal-like pattern of axial rotation, although less in magnitude than the normal knee. The normal axial rotation pattern occurred because the lateral condyle rolled in the posterior direction, while the medial condyle moved in the anterior direction. Interestingly, the magnitude of posterior femoral rollback and axial rotation for subjects in this study was similar in magnitude reported in previous studies pertaining to asymmetrical TKA designs. It is proposed that more patients be analyzed having this TKA implanted by other surgeons.

Effect of sagittal femoral component alignment on biomechanics after mobile-bearing total knee arthroplasty

BACKGROUND

Recently, there has been increasing interest in mobile-bearing total knee arthroplasty (TKA). However, changes in biomechanics with respect to femoral component alignment in mobile-bearing TKA have not been explored in depth. This study aims to evaluate the biomechanical effect of sagittal alignment of the femoral component in mobile-bearing TKA.

METHODS

We developed femoral sagittal alignment models with - 3°, 0°, 3°, 5°, and 7°. We also examined the kinematics of the tibiofemoral (TF) joint, contact point on the TF joint, contact stress on the patellofemoral (PF) joint, collateral ligament force, and quadriceps force using a validated computational model under a deep-knee-bend condition.

RESULTS

Posterior kinematics of the TF joint increased as the femoral component flexed. In addition, contact stress on the PF joint, collateral ligament force, and quadriceps force decreased as the femoral component flexed. The results of this study can assist surgeons in assessing risk factors associated with femoral component sagittal alignment for mobile-bearing TKA.

CONCLUSIONS

Our results showed that slight flexion implantation may be an effective alternative technique because of its advantageous biomechanical effect. However, excessive flexion should be avoided because of potential loosening of the TF joint.

Effect of insert material on forces on quadriceps, collateral ligament, and patellar tendon after rotating platform mobile-bearing total knee arthroplasty

BACKGROUND

There is a gradual increase in the number of patients for total knee arthroplasty (TKA), and TKA demonstrates reliable clinical outcomes. The orthopaedic biomaterials community continuously attempted over the past decades to improve the longevity of UHMWPE in TKA by using various improved technologies. Polyetheretherketone (PEEK) and carbon fiber reinforced-PEEK(CFR-PEEK) are suggested as potential tibial insert materials to replace UHMWPE in some applications. The aim of this study involves evaluating the biomechanical effects of UHMWPE and CFR-PEEK tibial materials on mobile-bearing TKA.

METHODS

The finite element (FE) model was obtained by conducting computed tomography and magnetic resonance imaging. The FE investigation included three types of loading conditions corresponding to the loads used in the experiments for FE model validation and model predictions under deep-knee bend loading conditions. We investigated forces on quadriceps, collateral ligament and patellar tendon with UHMWPE and CCFR-PEEK tibial insert materials under the deep-knee-bend condition.

RESULTS

Quadriceps force decreased with flexion for CFR-PEEK when compared to that for UHMWPE. A similar trend was observed in terms of the patellar tendon force. An opposite trend was observed in the collateral ligament. Medial collateral ligament force in the CFR-PEEK exceeded that in the UHMWPE, and lateral collateral ligament force in the UHMWPE exceeded that in the CFR-PEEK.

CONCLUSION

The CFR-PEEK represents an alternative insert material given its superior biomechanical effect after mobile-bearing total knee arthroplasty. However, a balance between the medial and lateral ligaments is considered as an important factor in the CFR-PEEK tibial insert due to its opposite biomechanical effect.

Effects of posterior condylar offset and posterior tibial slope on mobile-bearing total knee arthroplasty using computational simulation

BACKGROUND

Postoperative changes of the femoral posterior condylar offset (PCO) and posterior tibial slope (PTS) affect the biomechanics of the knee joint after fixed-bearing total knee arthroplasty (TKA). However, the biomechanics of mobile-bearing is not well known. Therefore, the aim of this study was to investigate whether alterations to the PCO and PTS affect the biomechanics for mobile-bearing TKA.

METHODS

We used a computational model for a knee joint that was validated using in vivo experiment data to evaluate the effects of the PCO and PTS on the tibiofemoral (TF) joint kinematics, patellofemoral (PF) contact stress, collateral ligament force and quadriceps force, for mobile-bearing TKA. The computational model was developed using ±1-, ±2- and ±3-mm PCO models in the posterior direction and -3°, 0°, +3°, and +6° PTS models based on each of the PCO models.

RESULTS

The maximum PF contact stress, collateral ligament force and quadriceps force decreased as the PTS increased. In addition, the maximum PF contact stress and quadriceps force decreased, and the collateral ligament force increased as PCO translated in the posterior direction. This trend is consistent with that observed in any PCO and PTS.

CONCLUSIONS

Our findings show the various effects of postoperative alterations in the PCO and PTS on the biomechanical results of mobile-bearing TKA. Based on the computational simulation, we suggest that orthopaedic surgeons intraoperatively conserve the patient's own anatomical PCO and PTS in mobile-bearing TKA.

Comparison of Kinematics in Cruciate Retaining and Posterior Stabilized for Fixed and Rotating Platform Mobile-Bearing Total Knee Arthroplasty with respect to Different Posterior Tibial Slope

Reconstructed posterior tibial slope (PTS) plays a significant role in kinematics restoration after total knee arthroplasty (TKA). However, the effect of increased and decreased PTS on prosthetic type and design has not yet been investigated. We used a finite element model, validated using in vitro data, to evaluate the effect of PTS on knee kinematics in cruciate-retaining (CR) and posterior-stabilized (PS) fixed TKA and rotating platform mobile-bearing TKA. Anterior-posterior tibial translation and internal-external tibial rotation were investigated for PTS ranging from -3° to 15°, with increments of 1°, for three different designs of TKA. Tibial posterior translation and external rotation increased as the PTS increased in both CR and PS TKAs. In addition, there was no remarkable difference in external rotation between CR and PS TKAs. However, for the mobile-bearing TKA, PTS had less effect on the kinematics. Based on our computational simulation, PTS is the critical factor that influences kinematics in TKA, especially in the CR TKA. Therefore, the surgeon should be careful in choosing the PTS in CR TKAs.

Tibial component coverage and rotational alignment accuracy after mobile-bearing total knee arthroplasty

BACKGROUND

Tibial component coverage (TCC) and tibial rotational angle (TRA) have been studied simultaneously in simulations, but not in clinical studies after total knee arthroplasty (TKA). The purposes of this study were (1) to evaluate TCC and rotational setting postoperatively in mobile-bearing TKA patients and (2) to compare the results with previously published simulation data.

METHODS

We prospectively examined 100 patients who underwent primary TKA using the LCS^® Total Knee System (LCS) posterior cruciate ligament-substituting prosthesis. Clinical outcomes, TCC (coverage area of the tibial component over the tibia), and TRA (relative to the femoral transepicondylar axis (TEA)) were assessed. Quantitative three-dimensional computed tomography was used to assess TCC and TRA. All values are expressed as median (25th percentile, 75th percentile) using minus (-) for internal and plus (+) for external rotation.

RESULTS

Hospital for Special Surgery scores improved from 46 (36, 50) preoperatively to 92 (90, 92) postoperatively. TRA showed a median divergence of - 2.0° (- 4.75°, + 2.74°). All knees were located within 10° of the TEA (range - 10.0° to + 9.7°). The median TCC of the knees was 82.7% (80.6, 84.7%), and there were no knees that hung over the tibial component in any direction.

CONCLUSIONS

The LCS prosthesis had good clinical outcomes, comparable TCC, and improved TRA as compared to previous reports, as all knees were located within 10° of the TEA. Simultaneous optimization of both TCC and TRA may contribute to the excellent long-term outcomes that have been observed with this system.

LEVEL OF EVIDENCE

Level II, Prognostic study.

What Postoperative Outcome Measures Link Joint Stability to Patient Satisfaction?

Total knee arthroplasty is a common, effective treatment for disabling arthritis of the knee. However, not all patients receiving total knee arthroplasty achieve what is considered an optimal outcome, and not all patients report high satisfaction with their results. Because the focus of attention increasingly turns to optimizing outcomes and utilization of healthcare resources, it is worthwhile to review the array of available tools and techniques for quantifying postoperative outcomes after knee arthroplasty and identify how these might be used to guide refinement of the treatment to achieve even better and more predictable outcomes. This review summarizes current methods for assessing postoperative outcomes on a biomechanical basis, organizes them into three complementary domains, and suggests how these domains must be linked to address the titular question.

Dynamic Knee Alignment and Collateral Knee Laxity and Its Variations in Normal Humans

Alignment of normal, arthritic, and replaced human knees is a much debated subject as is the collateral ligamentous laxity. Traditional quantitative values have been challenged. Methods used to measure these are also not without flaws. Authors review the recent literature and a novel method of measurement of these values has been included. This method includes use of computer navigation technique in clinic setting for assessment of the normal or affected knee before the surgery. Computer navigation has been known for achievement of alignment accuracy during knee surgery. Now its use in clinic setting has added to the inventory of measurement methods. Authors dispel the common myth of straight mechanical axis in normal knees and also look at quantification of amount of collateral knee laxity. Based on the scientific studies, it has been shown that the mean alignment is in varus in normal knees. It changes from lying non-weight-bearing position to standing weight-bearing position in both coronal and the sagittal planes. It also varies with gender and race. The collateral laxity is also different for males and females. Further studies are needed to define the ideal alignment and collateral laxity which the surgeon should aim for individual knees.

Rotational alignment of tibial components in mobile-bearing TKA: posterior substituted vs. PCL retaining

PURPOSE

The medial border of the tibial tubercle (MBTT) is one of the fixed anatomic landmarks for tibial component setting during total knee arthroplasty (TKA). In mobile-bearing TKA using a tibial cut first technique, the final tibial component rotation can be guided by the position it achieves following several flexion-extension cycles. In this study, tibial component angle (TCA) and tibial rotational angle (TRA) were determined in dependence of retention or resection of the posterior cruciate ligament (PCL).

METHODS

The TCA and TRA were examined in 206 patients who underwent primary TKA (PCL retaining: 104 knees, PCL substituting: 102 knees). The tibial component rotation was intraoperatively setting between the parallel to the axis of the most medial aspect of the tibial tubercle as the anterior anatomic landmark and the center of the tibial component as the posterior landmark at the maximum coverage with the osteotomized tibial plateau with its adjustment after several knee flexion-extension exercises. A postoperative quantitative three-dimensional computed tomography technique was used for measurements by a single observer.

RESULTS

The TCA showed a divergence of 0.21° external to the MBTT in the PCL-retaining design and 1.62° internal divergence in the PCL-substituting design. The TRA showed an internal divergence of 0.88° in the PCL-retaining design and an internal divergence of 2.12° in the PCL-substituting design. There were no significant differences between the two designs.

CONCLUSIONS

The MBTT might be regarded as a reliable landmark for obtaining an acceptable tibial rotational setting in mobile-bearing TKA despite PCL retention.

Does knee stability in the coronal plane in extension affect function and outcome after total knee arthroplasty?

PURPOSE

The aim of the present study was to clarify whether varus-valgus laxities under static stress in extension, femoral condylar lift-off during walking, and patient-reported outcomes after total knee arthroplasty (TKA) were correlated with each other.

METHODS

Ninety-four knees, which had undergone posterior-stabilized TKA, were analysed. The varus-valgus laxity during knee extension was measured using a stress radiograph. New Knee Society Score (KSS) questionnaires were mailed to all patients. Correlations between the values of stress radiographs and KSS were analysed. Additionally, continuous radiological images were taken of 15 patients while each walked on a treadmill to determine condylar lift-off from the tibial tray using a 3D-to-2D image-to-model registration technique. Correlations between the amount of lift-off and either the stress radiograph or the KSS were also analyzed.

RESULTS

The mean angle measured was 5.9 ± 2.7° with varus stress and 5.0 ± 1.6° with valgus stress. The difference between them was 0.9 ± 2.8°. Varus-valgus laxities, or the differences between them, did not show any statistically significant correlation with either component of the KSS (p > 0.05). The average amount of femoral condylar lift-off during walking was 1.4 ± 0.8 mm (medial side) and 1.3 ± 0.6 mm (lateral side). The amount of lift-off did not correlate with either varus-valgus laxities or the KSS (p > 0.05).

CONCLUSIONS

No correlations were found among varus-valgus laxities under static stress in extension, femoral condylar lift-off during walking, or patient-reported outcomes after well-aligned TKA. This study suggests that small variations in coronal laxities do not influence lift-off during walking and the patient-reported outcomes.

LEVEL OF EVIDENCE

IV.

Collateral ligament laxity in knees: what is normal?

BACKGROUND

Proper alignment and balancing of soft tissues of the knee are important goals for TKA. Despite standardized techniques, there is no consensus regarding the optimum amount of collateral ligament laxity one should leave at the end of the TKA.

QUESTIONS/PURPOSES

I asked (1) what is the collateral laxity in young healthy volunteers, and (2) is there a difference in collateral laxity between males and females.

METHODS

The femorotibial mechanical angle (FTMA) was measured in 314 knees in healthy volunteers aged 19 to 35 years. Subjects with a history of pain, malalignment, dysplasia, or trauma were excluded. Twenty-five knees were excluded because the hip center could not be acquired, and 22 were excluded because of a history of pain and trauma, leaving 267 knees for inclusion in the study. Of these, 155 were from men and 112 were from women. A validated method using a computer navigation system was used to obtain the measurements. A 10-Nm torque was used to stress the knee in varus and valgus at 0° extension and 15° flexion. An independent t-test and ANOVA were applied to the data to calculate any significant difference between groups (p<0.05).

RESULTS

The mean (SD) unstressed supine FTMA was varus of 1.2° (SD, 4°) in 0° extension and varus of 1.2° (SD, 4.4°) in 15° flexion (p=0.88). On varus torque of 10 Nm, the supine FTMA changed by a mean of 3.1° (SD, 2°) (95% CI, 2.4°-3.8°; p<0.001) in 0° extension and 6.9° (SD, 2.6°) (95% CI, 6.2°-7.7°; p<0.001) in 15° flexion. On valgus torque of 10 Nm, the FTMA changed by a mean of 4.6° (SD, 2.2°) (95% CI, 3.9°-5.3°; p<0.001) in 0° extension and 7.9° (SD, 3.4°) (95% CI, 7.1°-8.7°; p<0.001) in 15° flexion. The mean unstressed FTMA in 0° extension was varus of 1.7° (SD, 4°) in men and 0.4° (SD, 3.9°) in women (p=0.01). Differences in collateral ligament laxity were seen between men and women (p<0.001 for valgus torque and 0.035 for varus torque in 15° flexion). With valgus torque at 0° flexion, the supine FTMA change was valgus of 4.2° (SD, 2.0°) for men and 5.0° (SD, 2.4°) for women, while at 15° flexion the FTMA change was valgus 7.6° (SD, 3.6°) for men and 8.3° (SD, 3.2°) for women With varus torque at 0° flexion, additional varus was -3.0° (SD, 1.8°) for men and -3.3° (SD, 2.2°) for women, while at 15° flexion, varus was -7.0° SD, (2.5°) for men and -6.9° (SD, 2.8°) for women.

CONCLUSIONS

The collateral laxity in young healthy volunteers was quantified in this study. The collateral ligament laxity is variable in different persons. In addition, ligaments in women are more lax than in men in valgus stress.

CLINICAL RELEVANCE

This study was conducted on young, healthy knees. Whether the findings are applicable to arthritic knees and replaced knees needs additional evaluation. However the findings provide a baseline from which to work in the evaluation of arthritic knees and in the case of TKA.

No influence of coronal laxity and alignment on lift-off after well-balanced and aligned total knee arthroplasty

PURPOSE

In vivo fluoroscopic analyses have revealed the kinematics after total knee arthroplasty (TKA), including femoral condylar lift-off. This study asked whether differences in static varus-valgus laxity or coronal limb alignment after TKA affect lift-off under weight-bearing conditions. It was hypothesised that there is a correlation between coronal laxity or alignment and lift-off during walking.

METHODS

The current study analysed nineteen subjects undergoing cruciate-retaining TKA performed by the measured resection technique. The varus-valgus laxity at knee extension was measured using a 150 N stress radiograph. The mechanical axis was measured using a full-standing radiograph. Continuous radiological images were taken while the subject walked on a treadmill, and the images during single-leg stance were analysed to determine the lift-off using a 3D-to-2D image-to-model registration technique.

RESULTS

The average angle in varus/valgus stress was 6.8 ± 1.8°/6.6 ± 2.1°. No statistically significant differences were observed between the varus and valgus laxity. The average amount of lift-off was 0.7 ± 0.4 mm. The static varus-valgus laxity (n. s.) or the differences in the laxities (n. s.) on the stress radiograph did not influence lift-off. The weight-bearing ratio was achieved within the middle third of the knee in 90 % of subjects. Two outliers with valgus alignment (68 ± 1 %) demonstrated no significant difference in lift-off in comparison with the majority of the subjects (46 ± 9 %).

CONCLUSION

The static coronal laxity and alignment did not influence the lift-off under dynamic weight-bearing conditions after well-balanced and aligned cruciate-retaining TKA. Measured resection technique can produce sufficient coronal stability and alignment without significant lift-off during walking.

Abnormal axial rotations in TKA contribute to reduced weightbearing flexion

BACKGROUND

Previous in vivo fluoroscopy studies have documented that axial rotation for patients having a TKA was significantly less than those having a normal knee. In fact, many subjects having a TKA experience a reverse axial rotation pattern where the femur internally rotates with increasing flexion. However, no previous studies have been conducted to determine if this reverse axial rotation pattern affects TKA performance.

QUESTIONS/PURPOSES

The purposes of this study were: (1) Do normal and reverse axial rotation patterns of a TKA affect the maximum flexion angle postoperatively? (2) Does the axial rotation angle of the knee at maximum flexion during weightbearing impact the magnitude of the maximum flexion achieved in weightbearing?

METHODS

One hundred twenty patients having TKA, previously analyzed under in vivo conditions using fluoroscopy and a three-dimensional model-fitting software package, were further evaluated to determine if reverse axial rotation patterns limit weightbearing TKA flexion. In this retrospective cohort, we identified 58 patients who had a normal axial rotation pattern (greater than 15° normal rotation). Sixty-two patients experienced greater than 3° of reverse axial rotation, defined as internal rotation of the femur relative to the tibia.

RESULTS

Patients having a normal axial rotation achieved greater weightbearing knee flexion than those with reverse axial rotation (115° versus 109°, p = 0.02). Additionally, patients with greater than 3° of normal axial rotation at maximum flexion had more flexion than those with less than 3° of normal axial rotation at ending flexion (115° versus 107°, p < 0.001).

CONCLUSIONS

These findings show reverse axial rotation and a smaller magnitude of normal axial rotation reduce weightbearing knee flexion. This is likely the result of increased posterior movement of the lateral condyle and is an important consideration in future implant designs.

Does mobile-bearing knee arthroplasty motion change with activity?

BACKGROUND

The purpose of this study was to evaluate the effect of mobile-bearing implant design and activity on knee arthroplasty kinematics during three activities of daily living.

METHODS

In vivo kinematics were analyzed using 3D model registration from fluoroscopic images of non-weightbearing knee flexion-extension, weight-bearing squatting and stair activities in 20 knees in 10 patients with bilateral total knee arthroplasty. Each patient had one rotating-platform and one meniscal-bearing variant of the same prosthesis design.

RESULTS

Anteroposterior translations in meniscal-bearing knees were larger than those in rotating-platform knees for the different dynamic conditions. Meniscal-bearing knees showed more posterior femoral locations with activities that increased demand on the quadriceps. Condylar translations changed little in rotating-platform knees with different activities.

CONCLUSIONS

Activity dynamics can have a significant influence on knee kinematics, and have a greater effect on the kinematics of unconstrained meniscal-bearing prostheses than rotating-platform knee prostheses.

LEVEL OF EVIDENCE

Level II.

Flexion and extension laxity after medial, mobile-bearing unicompartmental knee arthroplasty: a comparison between a spacer- and a tension-guided technique

PURPOSE

In a mobile-bearing unicompartmental knee arthroplasty (UKA), stability is of utmost importance to promote knee function and to prevent dislocation of the insert. Gap balancing can be guided by the use of spacers or a tensioner. The goal of this study is to compare laxity of a tension-guided implantation technique versus a spacer-guided technique for medial UKA with a mobile bearing. Also clinical function was compared between the groups.

METHODS

The tension-guided UKA system (BalanSys™, Mathys Ltd, Bettlach, Switzerland) was compared with a retrospective group with a spacer-guided system (Oxford, Biomet Ltd, Bridgend, UK). A total of 30 tension-guided medial UKAs were implanted and compared with 35 spacer-guided medial prostheses. In both groups, valgus laxity was measured at least 4 months postoperatively in extension and 70° flexion using stress radiographs. Knee Society Scores (KSS) were obtained at the 6-month follow-up.

RESULTS

Valgus laxity in flexion was significantly higher in the tension-guided group compared with the spacer-guided group: 3.9° (SD 1.8°) versus 2.4° (SD 1.2°), respectively, P < 0.001). In extension, valgus laxity was significantly different: 1.8° (SD 1.0°) in the tension-guided group compared with 2.7° (SD 0.9°) in the spacer-guided group (P < 0.001). There was no significant difference between the KSS for the two groups (n.s.).

CONCLUSIONS

The tensor-guided system resulted in significantly more valgus laxity in flexion compared with the spacer-guided system. However, in extension, the situation was reversed: the tension-guided system resulted in less valgus laxity than the spacer-guided system. Clinically, there were no differences between the groups. The valgus laxity found with the spacer-guided system better approximates the valgus laxity values of the healthy elderly.

Posterior condylar offset does not correlate with knee flexion after TKA

BACKGROUND

Studies of medial and lateral femoral posterior condylar offset have disagreed on whether posterior condylar offset affects maximum knee flexion angle after TKA.

QUESTIONS/PURPOSES

We asked whether posterior condylar offset was correlated with knee flexion angle 1 year after surgery in (1) a PCL-retaining meniscal-bearing TKA implant, or in (2) a PCL-substituting mobile-bearing TKA implant.

METHODS

Knee flexion angle was examined preoperatively and 12 months postoperatively in 170 patients who underwent primary TKAs to clarify the effect of PCL-retaining (85 knees) and PCL-substituting (85 knees) prostheses on knee flexion angle. A quasirandomized design was used; patients were assigned to receive one or the other implant using chart numbers. A quantitative three-dimensional technique with CT was used to examine individual changes in medial and lateral posterior condylar offsets.

RESULTS

In PCL-retaining meniscal-bearing knees, there were no significant correlations between posterior condylar offset and knee flexion at 1 year. In these knees, the mean (± SD) postoperative differences in medial and lateral posterior condylar offsets were 0.0 ± 3.6 mm and 3.8 ± 3.6 mm, respectively. The postoperative change in maximum knee flexion angle was -5° ± 15°. In PCL-substituting rotating-platform knees, similarly, there were no significant correlations between posterior condylar offset and knee flexion 1 year after surgery. In these knees, the mean postoperative differences in medial and lateral posterior condylar offsets were -0.5 ± 3.3 mm and 3.3 ± 4.2 mm, respectively. The postoperative change in maximum knee flexion angle was -2° ± 18°.

CONCLUSIONS

Differences in individual posterior condylar offset with current PCL-retaining or PCL-substituting prostheses did not correlate with changes in knee flexion 1 year after TKA. We should recognize that correctly identifying which condyle affects the results of the TKA may be difficult with conventional radiographic techniques.

Changes in lower extremity 3-dimensional load-bearing axes before and after mobile-bearing total knee arthroplasty

This study evaluated changes in the lower extremity 3-dimensional load-bearing mechanical axes in the anteroposterior and mediolateral directions before and at 3 weeks after mobile-bearing total knee arthroplasty. The effects of the degrees of anteroposterior constraint of the designs on the location of the load-bearing mechanical axis at the knee joint level were also assessed. We evaluated 151 knees from 134 patients with 74 knees receiving meniscal bearing-type and 77 knees receiving rotating platform-type prostheses. In the mediolateral direction, both designs showed significant improvements, whereas in the anteroposterior direction, they revealed no improvements postoperatively and were worsened significantly in meniscal bearing type. Differences in the degree of bone and soft tissue involvement for the correction of alignment may explain the findings.

SAGITTAL PLANE KNEE LAXITY AFTER LIGAMENT RETAINING UNCONSTRAINED ARTHROPLASTY: A MATHEMATICAL ANALYSIS

Passive knee laxity is an important clinical measure to assess function after joint replacement. Clinical observations suggest that the use of minimally invasive surgical techniques in knee arthroplasty may affect the surgeon's ability to orient and position the prosthetic components accurately. Further, recent studies suggest that malplaced prosthetic components in ligament retaining unconstrained unicompartmental knee arthroplasty (UKA) can affect the ligament forces and, hence, the knee laxity. In the present study, a sagittal plane mathematical model of the knee with intact ligaments and unconstrained prosthetic components is used to analyze antero-posterior (A–P) knee laxity during passive flexion at different force levels. Also, the effects of errors in component placement are evaluated. The model calculations show a reasonable agreement with the experimental observations reported in literature. The results show that the A–P laxity during 0°–120° flexion first increases from 0° to about 30°, remains nearly constant for another 10° and then decreases somewhat linearly for higher flexion angles. Some errors in the placement of femoral component of the order of 1 mm can affect the knee laxity by nearly 3 mm in some flexion positions. The analysis has clinical relevance and suggests that the UKA requires close attention to component placement.

The gap technique does not rotate the femur parallel to the epicondylar axis

INTRODUCTION

In the analysis of painful total knee replacements, the surgical epicondylar axis (SEA) has become established as a standard in the diagnosis of femoral component rotation. It remains unclear whether the gap technique widely used to determine femoral rotation, when applied correctly, results in a rotation parallel to the SEA.

METHOD

In this prospective study, 69 patients (69 joints) were included who received a navigated bicondylar surface replacement due to primary arthritis of the knee joint.

RESULTS

In 67 cases in which a perfect soft-tissue balancing of the extension gap (<1° asymmetry) was achieved, the flexion gap and the rotation of the femoral component necessary for its symmetry was determined and documented. The femoral component was implanted additionally taking into account the posterior condylar axis and the Whiteside's line. Postoperatively, the rotation of the femoral component to the SEA was determined and this was used to calculate the angle between a femur implanted according to the gap technique and the SEA. If the gap technique had been used consistently, it would have resulted in a deviation of the femoral components by -0.6° ± 2.9° (-7.4°-5.9°) from the SEA. The absolute deviation would have been 2.4° ± 1.8°, with a range between 0.2° and 7.4°.

CONCLUSION

Even if the extension gap is perfectly balanced, the gap technique does not lead to a parallel alignment of the femoral component to the SEA. Since the clinical results of this technique are equivalent to those of the femur first technique in the literature, an evaluation of this deviation as a malalignment must be considered critically.

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 5.1mm and 7.0mm, 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.

Coronal plane knee laxity measurement: Is computer-assisted navigation useful?

INTRODUCTION

The goal of this observational study is to measure the physiological laxity of a knee, supposedly normal in the coronal plane, at 0 and 90° of flexion with a navigation system that can be used during total knee replacement.

HYPOTHESIS

The physiological laxity measured by this navigation system is different from the results already published using other measurement devices.

MATERIALS AND METHODS

Twenty patients consecutively operated on for an isolated anterior cruciate ligament injury were selected. Medial and lateral laxities at 0 and 90° of knee flexion were measured by the navigation system during cruciate replacement.

RESULTS

The mean medial laxity in extension was 3.6±1.2°. The mean lateral laxity in extension was 4.1±1.9°. The mean medial laxity at 90° of flexion was 2.1±1.2°. The mean lateral laxity at 90° of flexion was 3.7±1.2°. The medial and lateral laxities in extension were not asymmetric. The medial and lateral laxities at 90° of flexion were asymmetric. Medial laxities in extension and at 90° of flexion were asymmetric. Lateral laxities in extension and at 90° of flexion were not asymmetric.

DISCUSSION

The data collected in our study suggest, during total knee replacement, the following tolerable ligamentous balance: medial and lateral laxities in extension about 3°, medial laxity at 90° of flexion about 2°, and lateral laxity at 90° of flexion about 4°.

LEVEL OF EVIDENCE

Level IV. Prospective study.

In vivo knee laxity in flexion and extension: a radiographic study in 30 older healthy subjects

In order to determine how "tight" a total knee prosthesis should be implanted, it is important to know the amount of laxity in a healthy knee. The objective of this study was to determine knee laxity in extension and flexion in healthy, non-arthritic knees of subjects similar in age to patients undergoing a total knee arthroplasty and to provide guidelines for the orthopaedic surgeon in his attempt to restore the stability of an osteoarthritic knee to normal. Thirty healthy subjects (15 male, 15 female), mean age 62 (SD 6.4) years, were included in the study. For each subject one, randomly selected, knee was stressed in extension and in 70 degrees flexion (15 Nm). Varus and valgus laxity were measured on radiographs. The passive range of motion and active flexion was assessed. Mean valgus laxity in extension was 2.3 degrees (SD 0.9, range 0.2 degrees -4.1 degrees ). In extension mean varus laxity was 2.8 degrees (SD 1.3, range 0.6 degrees -5.4 degrees ). In flexion, mean valgus laxity was 2.5 degrees (SD 1.5, range 0.0 degrees -6.0 degrees ) and mean varus laxity was 3.1 degrees (SD 2.0, range 0.1 degrees -7.0 degrees ). Varus and valgus knee laxity in extension and in flexion were comparable. This study shows that the normal knee in this age group has an inherent degree of varus-valgus laxity. Whether the results of the present study can be used to optimise the total knee arthroplasty implantation technique requires further investigation.