Abstract
Post total knee arthroplasty, mid flexion instability can be described as a stable knee in full extension but as soon as knee starts bending instability is noticed and the knee becomes stable again at 90° of flexion. Mid flexion instability should not be confused with the true flexion instability. Such instability may be not be recognized in most cases because of subtleness of the nature of complaints of the patient. Soft tissue tension should be equal not only medio-laterally but also in antero-posterior alignment. The knee needs to be balanced in the complete arc of motion. To understand this it should be remembered that main stabilizer of the knee in extension is the posterior capsule and in flexion are the collateral ligaments.
Main factors contributing to Mid Flexion instability are:
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1.
Over release of anterior part of Medial Collateral Ligament (which is a stabilizer from 30° to 60° of motion).
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2.
Femoral-tibial articular geometry - Malposition of the implant in relation to the epicondyles so that collateral ligaments won't be isometric.
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3.
Over release of anterior part of Medial Collateral Ligament (which is a stabilizer between 30° to 60° of motion
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4.
Tibial post-femoral box geometry.
In a fixed flexion deformity, suitable posterior release should be matched with the collateral frame before taking extra-distal femoral cuts. Every 2 mm of additional distal femoral cut causes mid flexion instability of 2 to 3° as was seen in a cadaveric study. It is important to understand the interplay between posterior structures and collateral structures. Normally collateral structures have some laxity at 5° flexion but at 0° knees are locked mainly because of the tension of the posterior structures.
We have classified mid flexion instability in three types:
Type I: Over-released MCL and Normalised Posterior capsule
Type II: MCL Normal, but Posterior capsule is tight / insufficiently released and to balance this disparity distal femur cut is increased.
Type III: A Combination of above two conditions with MCL and Postero-medial Capsule both having laxity e.g. in a FFD with varus
It is a retro-prospective study. 411 patients with 600 knees were subjected to the study to assess mid-flexion instability in patients with primary Total Knee Arthroplasty. Follow was over a period of 5 years. Of the 600 TKA 60 were LCS prosthesis, 90 were PFC RP, 200 were PFC sigma and rest 250 were Stryker Scorpio. All patients were assessed by clinical and radiological evaluation. X-rays were taken in 0°, 30°, 60°. Arthrograms were also done to assess alignment of the joints. Fluroscopic studies were done in select few cases. Knee society score was noted for each patient and compared with pre-operative data.
Mid Flexion instability in a newer concept, the causes of which and further management protocols needs to be worked out. Mid Flexion instability is a failure to release the tight posterior capsule in a fixed flexion deformity. Over release of anterior MCL will result in mid flexion instability but in this situation knee may be unstable even at 90°.
