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SHAPE DRIVES STABILITY

Category Tags: Clinical Studies
7/15/16
It is well established that 20% of patients report they are not satisfied with the results of their TKA. One reason that many patients report is their knee does not feel “natural”. In fact, patients dissatisfied with the results of their TKA report that their knee does not feel normal at more than 2x the rate of satisfied 

patients.1One explanation for why patients report a knee that does not feel normal is the prevalence of mid-flexion instability. Instability in flexion has been shown to be a cause of pain and functional impairment following TKA.2 According to published data, 2-3 times as many patients with a TKR report somedegree of difficulty with activities relative to an age-matched control group with no previous knee disorders.3
One of the identified causes of mid-flexion instability is movement of the joint line over 5mm.4 When a flat distal cut is made and then an implant with symmetrical condyles is put on the patient, the naturally occurring femoral offset is altered. The same happens when a polyethylene insert with the same medial and lateral heights is used.

Instability leads to implant failure

Not only is instability a cause of functional limitations, it is also associated with a 3.7x increase in the risk of implant failure.5 In a series of 279 patients who had revision surgery within 5 years, instability was the 2nd leading cause of early failure.6

The patient-specific solution

By respecting the patient’s natural J-curves, corrected for deformity, Conformis replicates the femoral offset in the implant design. Dual-balancing™ offsetpoly inserts allow for broader contact area and the maintenance of those medial and lateral joint lines. The iTotal®, in an in vivo study of patient kinematics, better approximated natural kinematics compared to both modern and legacy off-the-shelf total knee implant designs and exhibited motion patterns more closely resembling a normal knee.7

One potential reason for this difference is that standard knee replacement designs don’t preserve the patient’s condylar geometry, and their medial and lateral joint lines. As a result, off-the-shelf implants, including single radius designs, have been shown to alter kinematics.8

https://conformis.wistia.com/medias/v06qray367?embedType=async&videoFoam=true&videoWidth=640

References
1: Noble, et al; Patient Expectations Affect Satisfaction with Total Knee Arthroplasty. Clinical Orthopaedics and Related Research; 2006, 452: 35-43
2: Pagnano, et al; Flexion Instability After Primary Posterior Cruciate Retaining Total Knee Arthroplasty. Clinical Orthopaedics and Related Research; 1998, 356: 39-46
3: Noble, et al; Does Total Knee Replacement Restore Normal Knee Function? Clinical Orthopaedics and Related Research; 2005, 431: 157-165
4: Whiteside, et al; The Influence of Joint Line Position on Knee Stability After Condylar Knee Arthroplasty. Clinical Orthopaedics and Related Research; 1990, 259: 146-156
5: Fang, et al; Coronal Alignment in Post-operative TKA: Just how important is it? Journal of Arthroplasty; 2009, 24 #2: 39-43
6: Fehring, et al; Early Failures in Total Knee Arthroplasty. Clinical Orthopaedics and Related Research; 2001, 392: 315-318
7: Kurtz, et al; In Vivo Kinematics for Customized, Patient Specific Vs. Two Generations of Traditional TKA During Various Activities. Presented at the British Association for Surgery of the Knee 2016 Annual Meeting.
8: Bull, et al; Component Rotation and Anterior Knee Pain after Total Knee Arthroplasty. Clinical Orthopaedics and Related Research; 2008, 466: 2491-2499

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