Patient-specific guide for revision of medial unicondylar knee arthroplasty to total knee arthroplasty: beneficial first results of a new operating technique performed on 10 patients

Abstract

Background and purpose: Unicompartmental to total knee arthroplasty revision surgery can be technically demanding. Joint line restoration, rotation, and augmentations can cause difficulties. We describe a new technique in which single-way fitting guides serve to position the knee system cutting blocks.

Method: Preoperatively, images of the distal femur and proximal tibia are taken using CT scanning. These images are used to create a patient-specific guide that fits in one single position on the contours of the bone and the prosthesis in situ. The guides are fixed with pins and then removed. The pins determine the position of the cutting blocks. 10 consecutive revisions were performed using this technique.

Results: All guides fitted well. 7 of 10 femoral prostheses were within the desired AP and sagittal angle ± 3°. However, 1 proximal tibia did not have enough bone stock on the medial plateau for adequate fixation of the guide, so conversion to intramedular referencing was performed. This was to be expected after the preoperative planning. All tibial components were within the desired AP angle ± 3° and 7 of 10 were within the desired sagittal angle. Hip-knee-ankle angle was within 0 ± 3° in 8 of 10 cases.

Interpretation: This new technique makes preoperative planning and execution of this plan during surgery less demanding. Problems such as the need for augmentations can be predicted at the preoperative planning. The instrumentation must be redesigned in order to make this technique work in cases where there is minimal bone stock present.

Figure 1.

Digital virtual plan of the operation to be performed, and actual postoperative radiographs.

Figure 2.

Single-way fitting position of guides on the patient’s individual anatomy.

Figure 3.

Illustration showing that there was no contact between either the cartilage of the lateral femoral condyle or the lateral tibial plateau and the guide.

Figure 4.

Repositioning of the guide after removal of the prosthesis using the three predrilled holes and insertion of the fourth pin.

Figure 5.

Pins were used to position the femoral cutting blocks to make bone resections for the new TKA.

Figure 6.

Preoperative planning shows that the medial anterior pin (blue line marked and red arrow) cannot be drilled into bone in a stable way.