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Concept of a Femur-First-Extension-Gap-Balancer for Optimized Manual Kinematic Alignment in Total Knee Arthroplasty
Author(s):
Calließ T. (Germany)
,
Calließ T. (Germany)
Affiliations:
Karkosch R.
,
Karkosch R.
Affiliations:
Windhagen H.
,
Windhagen H.
Affiliations:
Ettinger M.
Ettinger M.
Affiliations:
ESSKA Academy. Calliess T. May 9, 2018; 209543; P09-1962 Topic: Joint Replacement
Assoc. Prof. Tilman Calliess
Assoc. Prof. Tilman Calliess
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Abstract
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Objectives: Kinematic Alignment (KA) was introduced in total knee arthroplasty (TKA) with encouraging clinical results.
The basic principle of KA is to reconstruct the pre-arthritic anatomy of the femur and tibia with the prosthetic components, and thus to restore the physiological ligament tension of the knee without releases.
However, previous experiences have shown that (1) the pre-arthritic tibia surface can be hard to approximate and (2) with the use of standard prosthetic designs some compromises must be accepted that may affect the ligament tension. In the published surgical algorithms the tibia cut is stepwise adapted to achieve the aimed ligament isometry. However, there are currently no specific instruments available.
The objective of this paper was to develop and evaluate a ligament guided instrument to align the tibia cut based on the femoral component position to achieve a physiological balanced flexion and extension gap in KA TKA.

Methods: The constructed Femur-First-Gap-Balancer (FFB) is based on a modified standard femoral trail component. First, said femoral component is positioned in a manual technique to achieve KA as published by Howell et al. After removal of all relevant osteophytes, the knee is put to extension and the gap is broad to the natural tension by the use of spacer blocks. When a balanced extension gap is achieved the tibia cut is aligned parallel to the distal femur by the use of a newly designed device coupled onto the femur component. The tibia slope is adjusted by flexing the tibia.
This device and surgical protocol was evaluated on five cadaver knees. Medio-lateral and antero-posterior joint stability was evaluated in the native knee and with trail components in place using a force controlled robotic arm. The component position is evaluated on pre- and postoperative CT scans.

Results: With the use of our FFB and the use of a single radius prosthesis we were able to reconstruct the physiological joint isometry on the medial compartment throughout the whole range of motion. In the lateral compartment the physiological laxity in flexion was reproduced whereas the extension was symmetric to the medial side. Overall, the joint stability with the trail components tended to be slightly higher than in the native knee, except for the anterior drawer. The native femoral surface was restored accurate within 1mm deviation, whereas the tibia cut showed changes up to 2mm to the physiological surface.

Conclusions: The FFB appears to be an interesting concept to simplify, optimize and standardize the tibia alignment in KA TKA with respect to the soft tissue tension. However, this study represents a proof of principle on non-arthritic knees. Further evaluation on arthritic knees is necessary. Despite the successful reconstruction of a physiological soft tissue balance, one limitation of this approach is, that the surgeon has limited control on the resulting tibia varus position as this is solely driven by the soft tissue. This effect also needs further evaluation.

Keywords:
Total knee arthroplasty, gap balancing, kinematic alignment, femur first
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