Computer-assisted subcapital correction osteotomy in slipped capital femoral epiphysis using individualized drill templates

Abstract

Background: Subcapital osteotomy by means of surgical hip dislocation is a treatment approach offered for moderate-to-severe cases of Slipped Capital Femoral Epiphysis (SCFE). This procedure is demanding, highly dependent on the surgeon's experience, and requires considerable radiation exposure for monitoring and securing the spatial alignment of the femoral head. We propose the use of individualized drill guides as an accurate method for placing K-wires during subcapital correction osteotomies in SCFE patients.

Methods: Five CT scans of the hip joint from otherwise healthy patients with moderate-to-severe SCFE were selected (ages 11-14). Three dimensional models of each patient's femur were reconstructed by manual segmentation and physically replicated using additive manufacturing techniques. Five orthopaedic surgeons virtually identified the optimal entry point and direction of the two threaded wires for each case. 3D printed drill guides were designed specific to each surgical plan, with one side shaped to fit the patient's bone and the other side containing holes to guide the surgical drill. Each surgeon performed three guided (using the drill guides) and three conventional (freehand) simulated procedures on each case. Each femur model was laser scanned and digitally matched to the preoperative model for evaluation of entry points and wire angulations. We compared wire entry point, wire angulation, procedure time and number of x-rays between guided and freehand simulated surgeries.

Results: The guided group (1.4 ± 0.9 mm; 2.5° ± 1.4°) was significantly more accurate than the freehand group (5.8 ± 3.2 mm; 5.3° ± 4.4°) for wire entry location and angulation (p < 0.001). Guided surgeries required significantly less drilling time and intraoperative x-rays (90.5 ± 42.2 s, 3 ± 1 scans) compared to the conventional surgeries (246.8 ± 122.1 s, 14 ± 5 scans) (p < 0.001).

Conclusions: We conclude that CT-based preoperative planning and intraoperative navigation using individualized drill guides allow for improved accuracy of wires, reduced operative time and less radiation exposure in simulated hips.

Clinical relevance: This preliminary study shows promising results, suggesting potential direct benefits to SCFE patients by necessitating less time under anesthesia and less intra-operative radiation exposure to patients, and increasing surgical accuracy.

Fig. 1

Spatial alignment of the femoral head with respect to femoral neck in its anatomic state of one of the hips included in the study (a. anterior, b. superior) and after reduction (c. anterior, d. superior) following general guidelines of subcapital osteotomy in SCFE

Fig. 2

Virtual pre-operative planning in a representative case (a. anterior, b. lateral, c. superior). Each surgeon identified the optimal position for three k-wires in the three anatomical planes, one through the fovea and two uniformly distributed in the head using Mimics (d. Mimics interface)

Fig. 3

Data collection set-up; a. Surgeons used a c-arm for intra-operative validation, as needed; b. The contrast in radiodensity of casted bone models and the wires allowed for visualization of the bony contour and wire trajectory during the simulated surgeries

Fig. 4

Drilling platform for simulated surgeries (a. view from top, b. view from right). The platform consisted of a 3D printed fixed support and an interchangeable bone unit. The two parts are attached using screws

Fig. 5

Digital models of the femur and mold box; Each bone was digitally divided into two halves (a. anterior, b. posterior) to be later molded into urethane rubber (note the holes for alignment pins in the split surface that ensures secure registration to the base of mold box); c. two separate mold boxes were designed to account for male and female halves of the final rubber mold (note the difference in alignment pins)

Fig. 6

Bone models were created using urethane rubber mold and two different urethane casting resins; a. The two halves of the urethane rubber mold; b. example bone model cut in half to demonstrate the semi-rigid shell and porous foam mimicking real bone structure

Fig. 7

Registration surface for a patient-specific instrument is guided by surgical exposure of proximal femur following standard of care; a. anterior registration surface is selected to ensure clearance from head-neck junction, fit patient’s femoral-shaft anatomy (starting below the greater trochanter and ending above the foveal wire’s exit point); b. lateral registration surface accommodates wire distribution; c. superior registration surface relies on surgical access to femoral neck’s mid-line; d. overall registration surface; e. final instrument design with break-away notch to ensure easy removal; f. placement and use of a representative drill guides in a simulated surgery

Fig. 8

Topographic map of a representative casted bone used to verify shape consistency and accuracy in a. anterior and b. superior views

Fig. 9

Visual representation of drilling path error in anterior plane in a representative freehand simulated surgery (a) vs a guided simulated surgery (b). The black lines represent the trajectory of wires according to plan. The red lines represent the results from the simulated surgeries