Measurement of the migration of a focal knee resurfacing implant with radiostereometry

Figures & data

Figure 1. The Episeal articular resurfacing implant.

Figure 2. Blown-up schematic illustration of the markers measured on the implant. The distal part of the component was machined into a 3.0-mm hemisphere. A 1.0-mm tantalum marker (black solid circle) was glued to this hemisphere. The markers measured (red crosses) were: (1) manual measurement of the center (dotted circle) of the 3.0-mm hemisphere (hemisphere RSA); (2) manual measurement of the distal tip of the implant (tip RSA); and (3) semi-automated measurement of the tantalum marker (standard RSA). The radiograph shows actual measurements of tip RSA (marker 131) and hemisphere RSA (marker 121) in a patient.

Table 1. Precision of standard, tip, and hemisphere RSA

	Standard RSA				Tip RSA				Hemisphere RSA
	2.09 × SD	mean	min	max	2.09 × SD	mean	min	max	2.09 × SD	mean	min	max
Translation (mm)
x	0.02	–0.01	–0.05	0.09	0.14	0.07	–0.07	0.19	0.18	0.03	–0.08	0.19
y	0.04	0.02	–0.01	0.07	0.19	0.02	–0.13	0.21	0.12	–0.04	–0.14	0.09
z	0.09	0.01	–0.09	0.12	0.20	0.08	–0.15	0.23	0.33	0.05	–0.25	0.36
Total migration (mm)
MTPM	0.11	0.14	0.0	0.19	0.20	0.23	0.09	0.46	0.18	0.26	0.08	0.37

Table 2. Accuracy of tip and hemisphere RSA

	Tip RSA				Hemisphere RSA
	2.26 × RMS	mean	min	max	2.26 × RMS	mean	min	max
Translation (mm)
x	0.10	0.00	–0.11	0.05	0.09	0.02	–0.04	0.09
y	0.08	–0.02	–0.06	0.03	0.17	–0.03	–0.14	0.11
z	0.17	0.03	–0.06	0.15	0.19	0.03	–0.11	0.14
Total migration (mm)
MTPM	0.20	0.03	–0.06	0.15	0.23	0.09	0.05	0.17