Accuracy of bone resection in total knee arthroplasty using CT assisted-3D printed patient specific cutting guides

Abstract

Introduction: We conducted this study to determine if the pre-surgical patient specific instrumented planning based on Computed Tomography (CT) scans can accurately predict each of the femoral and tibial resections performed through 3D printed cutting guides. The technique helps in optimization of component positioning determined by accurate bone resection and hence overall alignment thereby reducing errors.

Methods: Prophecy evolution medial pivot patient specific instrumented knee replacement systems were used for end stage arthrosis in all consecutive cases over a period of 20 months by a single surgeon. All resections (4 femoral and 2 tibial) were measured using a vernier callipers intraoperatively. These respective measurements were then compared with the preoperative CT predicted bone resection surgical plan to determine margins of errors that were categorized into 7 groups (0 mm to ≥2.6 mm).

Results: A total of 3618 measurements (averaged to 1206) were performed in 201 knees (105 right and 96 left) in 188 patients (112 females and 76 males) with an average age of 67.72 years (44 to 90 years) and average BMI of 32.3 (25.1 to 42.3). 94% of all collected resection readings were below the error margin of ≤1.5 mm of which 90% showed resection error of ≤1 mm. Mean error of different resections were ≤0.60 mm (P ≤ 0.0001). In 24% of measurements there were no errors or deviations from the templated resection (0.0 mm).

Conclusion: The 3D printed cutting blocks with slots for jigs accurately predict bone resections in patient specific instrumentation total knee arthroplasty which would directly affect component positioning.

Figure 1

Femoral and tibial cutting block mounted on the distal femur and on the proximal tibia respectively. These are company manufactured patient specific femoral cutting blocks based on CT scan created virtual bone models. Multiple contact points on the femoral cutting block and the tibial cutting block, along with markings allow accurate positioning and rotational alignment improving accuracy of the cuts and femoral and tibial rotation respectively. There are slots for external jigs to double check slope and tibial base plate rotation.

Figure 2

The PSI cutting block placed accurately on the femoral surface and pinned. An intraoperative image showing the patient specific cutting block placed accurately on the distal femoral surface − flexion and rotation checked. The block contains the cutting jig in situ. The block contains information related to the surgery.

Figure 3

Measurement of the medial and lateral distal femoral condyle and posterior condylar cuts. Sequential pinning of the block was carried out very carefully with 2 distal pins (through the 3D block) followed by 2 anterior femoral pins (through the in situ jig). The oblique anterior femoral pin was placed last to prevent any movement of the 3D block whilst resecting. Distal femoral resections were then done through the in situ jig. The posterior condylar cuts were done through the appropriate standard 4:1 in block as per plan. The largest thickness of bony prominence was recorded by the senior surgeon using Vernier Calipers for each resected medial and lateral distal femoral condyle. Three separate measurements − horizontal, vertical and diagonal were made, and the average measurement was recorded independently.

Figure 4

The tibial block was seated as per surgical plan and pinned. This is an intraoperative image showing the patient specific cutting block placed on the proximal tibial surface which perfectly sat, once soft tissue was dissected off the bony surfaces for the contact points of the block. Tibial rotation and slope was double checked by extra-medullary alignment guide through the 3D block and the oblique pin placed before the cuts were made.

Figure 5

Tibial cut measured from the corresponding points of the plan and recorded for medial and lateral surfaces of tibial plateau. The cuts were measured by the senior surgeon using Vernier Calipers from the corresponding points on the plan (marked with a dot to ensure accuracy) and recorded for medial and lateral surfaces of tibial plateau. Three separate measurements were averaged per resection.