Abstract
Introduction
The introduction of the Stanmore Implants Savile Row mobile-bearing UKA procedure in July 2011 marked a world first – the use of a patient-specific knee implanted with robotic technology – the Sculptor Robotic Guidance Arm (RGA). This union gives a truly personalised solution by designing an implant for each patient based upon preoperative CT data and using Sculptor RGA to prepare the bone accurately so that the implant is correctly positioned as planned. The purpose of this study is to evaluate the accuracy of Sculptor RGA both in-vitro and in-vivo. We report on the accuracy of our first clinical procedures.
Methods
In-vitro:
CTs of plastic-bones were used to create plans for Sculptor RGA, establishing a relationship between the implant position and plastic-bone (planned-transform). Sculptor RGA was then used to prepare bones for 16 UKA implants mimicking the clinical set-up. The implants were placed in the prepared bones without cement. A coordinate-measuring-arm was used to register a)the bone, and b)the implant in relation to the bone (achieved-transform). The difference between planned-and-achieved transforms gives the error in implant position.
In-vivo:
Preoperative CTs of 8 OA patients, acquired using the low-dose Imperial Knee CT protocol, were used to plan the position and the shape of the patient-specific implants. Intra-operatively, Sculptor RGA was used to register and prepare the bone and the implants were cemented in place. Post-operative CTs were also acquired. Two techniques were used to measure planned-to-achieved positions of the implants: 1). Preoperative-to-postoperative CT image registration followed by extraction of the achieved implant position and comparison with the plan, 2). Surface-to-surface registration of bone-models segmented from the preoperative and postoperative CTs followed by extraction of the achieved implant position and comparison with the plan.
Results
For the in-vitro tests, femoral RMS errors averaged 0.8 mm and 1.6° and tibial RMS errors averaged 1.2 mm and 1.6°. For the clinical cases, femoral RMS errors averaged 1.2 mm and 2.6° and tibial RMS errors averaged 1.3 mm and 2.4°.
Discussion
The planned-versus-achieved errors are lower for the in-vitro cases compared to the clinical cases. This is partly due to the increased ease of registration in-vitro and also due to the error introduced during cementing in-vivo, which has been reported to give average errors of 2° in UKA1. The clinical results also compare favourably with previously published results for robotic UKA: Conditt et al report 1.4 mm and 2.6° for the femur and 1.2 mm and 2.1° for the tibia2.
We have previously reported on the accuracy of the predecessor to Sculptor RGA in a study implanting an off-the-shelf UKA prosthesis with the bones clamped and comparing this to the manual technique3. In that study the error in the robotic technique averaged femur: 1.0 mm and 2.6°, tibia: 1.1 mm and 2.5° compared to femur: 2.7 mm and 5.5°, tibia: 2.5 mm and 5.3° in the manual group.
Conclusion
This study has shown that the combination of Sculptor RGA and a patient-specific knee results in accurate and reproducible implant placement.
