Abstract:

Purpose. To estimate how much of the inaccuracy of correction with a hexapod fixator may result from errors made by the patient or caregiver during self-performed strut adjustment.

Methods. A review of the literature on correction accuracy, patient burden and systems supporting strut adjustment. This was followed by a kinematic model of a hexapod with 180 mm rings. The model converts strut length errors into residual deformity at the CORA and at a point 250 mm distal to it. Error types and the frequency of scale-reading errors (6% of readings deviating by 2–3 mm) were taken from published data [1]. An absolute schedule, in which the patient sets a length from a table, was compared with an incremental schedule, in which the patient performs a specified number of steps.

Results. None of the identified studies reports the frequency of patient errors during adjustment, although several cite them as a cause of inaccuracy [1–3]. In the model, a 1 mm error in one strut produced a mean of 0.40° of angulation and 0.95 mm of displacement at the CORA. Turning a strut the wrong way or swapping struts produced about 1° and 2–2.5 mm. With the absolute schedule, errors do not accumulate: p95 was 1.2° and 3.0 mm. With the incremental schedule, errors add up. With 2% erroneous adjustments and 120 adjustments per strut, p95 reached 2.6° and 6.2 mm, and 9.6 mm at 250 mm from the CORA.