Summary

This study validated the accuracy of a novel process designed to fabricate anatomically correct cartilage grafts in the weight bearing surfaces of human cadaver knees using MRI and 3D printing technology.

Abstract

Introduction

Limited treatment strategies exist for large cartilage defects in non-arthritic patients, and the ideal treatment strategy for these defects remains unknown. The specific aim of this project was to develop a patient specific, anatomically correct graft for cartilage restoration using MRI data and 3D printing technology, and to validate the accuracy of this novel process designed to fit the native contour of the human knee. Our hypothesis was that a custom-made anatomic graft would demonstrate better fit compared to a generic flat graft.

Methods

Four focal cartilage defects (FCDs) were created in paired human cadaver knees age <40 in the weight bearing surface of the 1) trochlea, 2) lateral facet of patella 3) medial femoral condyle, and 4) lateral femoral condyle of each knee. MRIs were obtained of each knee, and anatomic grafts were designed for the left knee as an experimental group, and generic flat grafts were printed for the right knee as a control group. All grafts were 3D printed with polylactic acid (PLA). Grafts were implanted into corresponding defects and fixed using tissue adhesive. After implantation, repeat MRI was obtained for visualization of graft fit.
The primary outcome was accuracy of a novel method for 3D printing individualized, anatomically shaped grafts, as measured on MRI. Graft step-off was measured as the distance between the surface of the graft and the native cartilage surface in a direction perpendicular to the subchondral bone. Graft contour was measured as the gap between the undersurface of the graft and the subchondral bone in a direction perpendicular to the joint surface. Measurements were performed every 1.8mm in two planes (n=18 for each femoral condyle and n=11 each for patella and trochlea) and recorded in millimeters. A students t-test was performed to compare means between groups.

Results

Anatomic experimental grafts corresponding to FCDs in the medial femoral condyle, lateral femoral condyle, patella, and trochlea were successfully 3D printed and implanted using MRI data. Graft step-off was significantly better for the anatomic grafts compared to the generic grafts in the medial femoral condyle (0.0+/-0.2 vs. 0.7+/-0.5, p<0.001), lateral femoral condyle (0.1+/-0.3 vs. 1.0+/-0.2, p<0.001), patella (-0.2+/-0.3 vs. -1.2+/-0.4, p<0.001), and trochlea (-0.4+/-0.3 vs. 0.4+/-0.7, p=0.003). Graft contour was significantly better for the anatomic grafts in the lateral femoral condyle and the trochlea. The anatomic grafts had an observed maximum step-off of -0.9mm and maximum contour mismatch of 0.8mm. The generic grafts had an observed maximum step-off of -1.7mm and maximum contour mismatch of 2.2mm.

Conclusions

This study provided validation of a process designed to fabricate an anatomically correct cartilage graft using MRI and 3D printing technology. Anatomic grafts demonstrated superior fit compared to generic flat grafts. Joint incongruity is clinically undesirable, and the mean anatomic graft step-off was less than 1mm in all FCDs. Mean graft contour was also less than 1mm in all FCDs, demonstrating excellent anatomic design and fit. Further research is needed to implement anatomic, biofunctionalized grafts in a large animal model.