Validation and precision of model-based radiostereometric analysis (MBRSA) for total ankle arthroplasty

The goal of this study was to design a RSA marker insertion protocol to evaluate the stability of the bone-implant interface of a TAA prosthesis, and to validate that this marker insertion protocol can be combined with MBRSA technology to provide clinically adequate precision in assessing the micromotion of the TAA prosthesis.

The Mobility™ Total Ankle System was used in this study. A marker placement protocol was developed with a Phantom Protocol. The Improved Marker Placement Protocol was used in 20 patients. Postoperative RSA double exams were taken. Condition Numbers (CN) were used to assess the marker distribution. The system precision was defined as the standard deviation of the double exams (MTE, MRE). MBRSA software was used to evaluate the double exams.

The RSA marker insertion technique for the 20 {\it in vivo} cases provided satisfactory results. CNs in all subjects but one were below 50 mm(-1) and implied a desirable marker configuration. The tibial sphere MTE was 0.07 mm and the talar was 0.09 mm. The talar MRE was 0.51 degrees.

The system precision for these {\it in vivo} TAA implants was within the normal range identified by RSA studies, and comparable to the existing TAA RSA studies. This study demonstrated a reliable RSA marker insertion technique in both the tibia and talus. The study confirms that the insertion and MBRSA technique allows the typical high precision demonstrated in other RSA studies (standard deviation less than or equal to 0.25 mm or 0.6 degrees).

This method may allow more accurate assessment of prosthetic subsidence clinically.