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Dr. Thomas Luyckx AZ Delta Hospital, Roeselare, Belgium

Restoring the joint line is a key consideration when performing total knee arthroplasty (TKA) due to the impact of optimal joint line positioning on good post-operative functional outcomes.

The effects of raising the joint line on patient kinematics remain a significant subject of modern research. Instability has become the most prevalent cause of early revisions after TKA (1).  
Our study specifically looked at the impact of the level of the joint line on the mid-flexion stability/ laxity. This stability is mainly provided by the superficial medial collateral ligament (sMCL) and is frequently overlooked as the knee is only assessed at 0° and 90° during surgery.  However the stability in the mid-flexion range is critical to our patients because during most daily activities the knee is not loaded in full extension but in mid-flexion (2,3). In primary TKA there is a susceptibility to raise the joint line in both flexion and extension (4, 5, 6, 7)

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In extension, failure to acknowledge and account for distal femoral wear, or increasing the depth of resection of the distal femur in cases of fixed flexion contracture, will result in a raised joint line (8, 9, 10). In flexion, most instrumentation systems rotate the femoral cutting block around a central pivot point, thus resecting more bone medially.

As a result when the prosthesis is positioned the medial joint line has been raised both distal and posterior; this more proximal and anterior position of the femoral implant is compensated for by using a thicker polyethylene insert to provide a stable joint in full extension and at 90° of flexion. However the relationship between joint line elevation and the isometry of the sMCL, thus mid-flexion stability is not well quantified.

Method: Ten fresh, frozen cadavers were implanted with a posterior stabilised (PS) Unity Knee™ prostheses using the corresponding instrumentation system, combined with computer navigation software. Joint laxity was examined by measuring the coronal plane stability at full extension, 30°, 60°, 90° and 120° of flexion by applying an instrumented varus and valgus torque.

Position TKA0: The specimens were randomised into two groups; five for mechanical alignment and five for kinematical alignment. In the mechanical alignment group, the femoral component was positioned at 0° to the mechanical axis in coronal alignment, and in 3° of external rotation relative to the posterior condylar line in the axial plane. The distal femoral and proximal tibial resections for the kinematic group were performed parallel to the native knee joint line, 0° external rotation applied in the axial plane. The trial components were positioned in the joint and the knee capsule closed in order to repeat the joint laxity measurements for the trial knee. The distal femoral and proximal tibial resections for the kinematic group were performed parallel to the native knee joint line, 0° external rotation applied in the axial plane. The trial components were positioned in the joint and the knee capsule closed in order to repeat the joint laxity measurements for the trial knee.

Position TKA2: After the analysis was completed, the trial components were removed and using navigation an additional 2mm of bone was resected from the distal and posterior femur. In order to stabilise the knee in flexion and extension, a 2mm thicker polyethylene component was used. As the above, the knee capsule was closed and the joint laxity measurements were completed as above.

Position TKA4: Finally an additional 2mm (total 4mm) was then resected using navigation from the distal and posterior femur. Stability was maintained by using a 2mm further thicker polyethylene component and joint laxity measurements repeated. For all the above positions, in the cases where the fixation of the trial component was insufficient, a Unity Knee™ implant was cemented in place to stabilise the knee.

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Figure 1: Mean coronal plane joint laxity in degrees is presented for each flexion angle. Results for the native knee and the TKA with the restored medial joint line are shown (TKA0). No statistical significant differences were noted between the native knee and the TKA0 position. Error bars indicate the standard deviation.

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Figure 2: Mean coronal plane joint laxity in degrees is presented for each flexion angle. Results for the TKA with the restored joint line (TKA0), the 2mm (TKA2) and 4mm (TKA4) raised joint line are shown. No statistical significant differences were noted between the three groups in extension or at 90° and 120° of flexion. However, a significant increase in coronal plane laxity was noted at 30° and 60° of flexion.

Results/Discussion: As the control in the study, the native knee joint stability was assessed and shown in figure 1 there was a gradual increase in laxity from full extension to 120° flexion. 

The same laxity pattern was also witnessed in the TKA0 position where the medial joint line had been maintained, which is consistent with findings of previous authors (11, 12). As such, the implant perfectly reproduced the natural joint stability throughout the range of motion. After raising the joint line, when in full extension, no difference in laxity was observed for both TKA2 and TKA4 positions.

However, when moved into mid flexion, a significant increase in coronal plane laxity occurred in the 2mm and 4mm raised joint line knees. The initial distal recut of 2mm (TKA2) increased the coronal plane laxity by an average of 64% at 30° flexion and 51% at 90° flexion.

Raising the joint line by 4mm (TKA4) had an even more significant effect on the mid flexion laxity, increasing 111% and 95% at 30° and 60° respectively, from the control position (TKA0). A linear correlation was found between the level of the raised joint line and the coronal plane stability; every millimetre rise in joint line level results in a 31% increase in coronal plane laxity at 30° and a 25% increase at 60°. Between the two groups, we found there was no effect of the mechanical vs. kinematic orientation of the joint line on stability.

Therefore the restoration of the medial joint line both distal and posterior is the fundamental criterion to facilitate normal joint laxity. In conclusion, our results indicate that restoration of the medial joint line reproduces normal sMCL isometry and joint stability after TKA.

Raising the joint line causes significant mid-flexion instability and tightness in deeper flexion despite a well-balanced flexion and extension gap. Further research is needed to prove that this concept does indeed improve joint stability and clinical outcome after TKA.

The Unity Knee™ Femoral Sizer instrument is a medial referencing rotational guide designed to facilitate medial joint line preservation throughout range of motion, restoring sMCL isometry when applying external rotation.

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1. Schroer WC, et al. J. Arthroplasty. 2013

2. Kettelkamp DB, et al. Bone Joint Surg 1970

3. Laubenthal KN, et al. Phys. Ther. 1972

4. Cope MR, et al. J Arthroplast. 2002

5. Ensini A, et al. Sports Traumatol, 2012

6. Kawamura H, et al. J Orthop Sci. 2001

7. Snider MG, et al. J. Arthroplasty. 2009

8. Bellemans J, et al. Clin. Orthop. Relat. Res. 2006

9. Cross MB, et al. Knee. 2012

10. Insall JN, et al. Clin. Orthop. Relat. Res. 1985

11. Van Damme G, et al. J. Bone Joint Surg. Am. 2005

12. Hunt NC, et al. J. Orthop. Res. 2014