Abstract
Introduction
Both navigation and instrumented bone referencing use unreliable intraoperative landmark identification or fixed referencing rules which don't reflect patient specific variability. PSI, however, lacks the flexibility to adapt to soft tissue factors not known during preoperative planning, in addition to suffering error from guide fit. A novel method of recreating surgical cut planes that combines preoperative image based identification of landmarks and planning with intraoperative adjustability is under development. This method uses an intraoperative 3D scan of the bone in conjunction with a preoperative CT scan to achieve the desired cuts and so avoids issues of intraoperative identification of landmarks.
Method
During TKA surgery, a reference device is placed on the exposed femur. The device is used to position a target block which is pinned to the bone (see Figure 1). The condyles and target block are then scanned, the process taking a second to complete. This 3D scan is filtered to remove extraneous bodies and noise leaving only the bony geometry and target block (see Figure 2). The scan is then reconciled to the known bone geometry taken from preoperative CT scans. A cutting block is then fixed to the target block with a reference array visible to the camera attached. Pre-planned cut planes on a computer model of the bone are compared to the position and configuration of the distal cutting guide. Software guides the surgeon in real-time on the necessary configuration changes required to align the cutting block. The cut is performed on the distal femur, the cutting guide removed from the target-block, and a second scan performed. The software repeats the filtering and alignment processes and provides the surgeon with data on how closely the performed cut matches the alignment planned.
Results
Two patients underwent this method alongside traditional alignment techniques. The initial 3D scan of the distal femoral condyles of the patients was matched to their corresponding CT scans. The first case had a mean error of 0.65 mm with 85% of errors falling below a magnitude of 1.16 mm and 58% falling below the case mean (see Figure 3). The second case had a mean error of 0.39 mm with 84% of errors falling below 0.70 mm and 60% falling below the case mean. It should be noted that the error introduced was due to the omission of soft tissue such as the PCL in the CT scan. Exposed bone portions of the scan geometry matched well with the CT scan, with error magnitudes significantly below the mean.
Discussion
The ability to obtain useful surgical alignment using preoperatively identified landmarks, alongside the small space requirements of a modern 3D scanner is sharply contrasted against the large space requirements and need for intraoperative probing of traditional navigation systems. Likewise, the use of preoperative planning and landmark identification to overcome intraoperative data capture variability mirrors that of PSI, but allows for potentially much greater accuracy of execution as the issue of guide fit and topology variation is avoided while intraoperative flexibility is maintained.
