Decreased bone-mineral density of as much as 20% has been observed after knee ligament injury, and this factor also may contribute to the increased risk of fracture after anterior cruciate ligament reconstruction because of decreased bending strength in the distal part of the femur28,30. When the area of the osseous defect is subjected to tensile stress, as it was when our patient sustained the knee extension injury, the load strength of the already vulnerable posterior aspect of the distal part of the femur is reduced even further23,24. However, as the bone in this anatomic region is predominantly under compression loading, the likelihood of fracture development and crack propagation is decreased, which may explain why femoral fracture does not occur more frequently after arthroscopic anterior cruciate ligament reconstruction.
As the stress concentration around osseous defects has been shown to decrease after eight to twelve weeks of osseous remodeling24, the predisposition for femoral fracture after anterior cruciate ligament reconstruction would be expected to decrease. However, healing of the femoral tunnel has been shown to be delayed by the exposure to biologic factors from the joint31. A previous case report on a patient in whom a fracture occurred through the femoral tunnel two years after anterior cruciate ligament reconstruction32 suggested that the stress-concentration effect of the femoral tunnel continues for a prolonged period after surgery.
Bone tunnel enlargement after anterior cruciate ligament reconstruction is well documented and has been reported to occur in as many as 68% of cases33. The etiology of this phenomenon is not completely understood, but it is thought to be related to a combination of multiple biological and mechanical factors34 and our understanding of the clinical relevance of bone tunnel enlargement is still evolving2,33,34. Previous experimental studies have shown that the breaking strength of bone decreases in direct proportion to the size of an osseous defect22. On the basis of these findings, the case of our patient suggests that enlargement of the femoral tunnel may have further decreased the mechanical fracture resistance. It has also been suggested that bone tunnel enlargement increases the risk of tibial plateau fracture after anterior cruciate ligament reconstruction6,19. Given the frequency of anterior cruciate ligament reconstruction and the high frequency of bone tunnel enlargement, the potential predisposing effect of this phenomenon toward a fracture of the distal part of the femur needs to be considered. Use of a more oblong drill-hole may help to reduce the stress concentration around the defect and help to reduce the fracture risk26.
Anatomic open reduction of the fracture is critical in order to avoid premature arthritis. In the case of our patient, we were also able to maintain the graft in the isometric position. Fracture fixation was successfully achieved with minimal postoperative morbidity, early functional recovery, and the return of a full range of motion. In contrast, other authors have reported continued loss of knee motion or ligamentous instability after distal femoral fracture fixation7,9,11. The intraoperative stability of the primary anterior cruciate ligament graft in our patient obviated the need for revision anterior cruciate ligament reconstruction. If anatomic fracture fixation does not maintain graft function, removal of the primary anterior cruciate ligament graft with bone-grafting of the enlarged bone tunnel can be performed at the time of fracture fixation to facilitate revision anterior cruciate ligament reconstruction at a later time.
In conclusion, supracondylar femoral fracture through the femoral bone tunnel is a serious complication of endoscopic anterior cruciate ligament reconstruction. The case of our patient suggests that bone tunnel enlargement may contribute to an increased risk of distal femoral fracture due to stress concentration around the femoral tunnel. The exact magnitude of the mechanical weakening and the postoperative duration of the increased fracture risk are unknown, and additional study is necessary to address these questions. If marked bone tunnel enlargement is observed, particularly in combination with other factors such as posttraumatic osteopenia, activity modification that reduces tensile force on the posterior aspect of the distal part of the femur may reduce the risk of distal femoral fracture after anterior cruciate ligament reconstruction. ▪
The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
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