JSUR Journal

Figure 6: A) Plain radiograph depicting integrity of the TKA revision with stem 24 weeks post-op. B) Plain radiograph showing a distal femoral fracture near the bone-implant interface and subsequent cement bolt failure just 5 days after the 24-week post-op radiograph.

Surgery/Phase 4: rTKA #2 and Failure

The patient was then referred to another surgeon who used a different brand of revision implant and a component with a larger diameter and robust stem. The patient underwent a second rTKA at an outside hospital and received a Vanguard SSK Constrained Knee, which was constrained in a cast for 4 weeks to ensure varus-valgus alignment. 7 weeks post-op, the patient returned for manipulation under anesthesia for arthrofibrosis and 16 weeks later was found to have incurred a femoral fracture along the stem of the femoral TKA component (Figure 7). The patient was told to refrain from cycling. The patient reported again, however, having returned to competitive cycling following cast removal and, after 16 weeks, started to have severe pain.

Figure 7: Plain radiograph depicting post-operative pre and post-midshaft femoral fracture around the stem implant from TKA revision.

Mechanical Modeling of Surgery/Phase 4 Failure

The fracture leading to phase 4 surgery was unlikely and not due to periprosthetic malfunction or operative error. The patient’s cycling, despite physician advice, likely produced the forces needed to fracture the femur around the stem implant. Previous studies have shown that cyclists weighing 60 kg experience a compressive force of 700-800 N in the tibiofemoral joint during pedaling, with a maximum of 910 N during a hill-climbing stance. [6,7] However, these forces alone are insufficient to fracture the femur. In a biomechanical experiment on sawbone models at the University of Illinois at Chicago, 1236.68 N was required to fracture the femur, approximately 55% more than seated pedaling and 36% more than hill climbing (Figure 8). The patient’s prior malunion, causing internal rotation of the distal tibia, added rotational forces that contributed to the fracture.

Figure 8: Saw-bone model with peri-prosthetics placed under axial load demonstrated mid-shaft fracturing at 1236.68 N of force.

Surgery/Phase 5: DFR and Failure

After the second rTKA failed, a DFR with a hinge was pursued at the outside hospital (OSH). The patient was advised not to exercise aggressively with this new implant, yet he continued to participate in competitive cycling. After six weeks, the patient returned for evaluation of leg pain, and it was discovered that the DFR had broken along the proximal metal stem (Figure 9).

Figure 9: Plain radiograph depicting AP of post-operative DFR and subsequent mid-stem fracturing at the site of the blue indicator. Surgery/Phase 6: Distal Femur Locking Plate and Failure

The femoral component of the DFR was replaced and reinforced with a femoral fracture plate with an allograft. After 12 weeks, the patient returned to the OSH and was found to have had nonunion of the proximal femoral osteotomy. Further imaging evaluation also revealed that the patient had unscrewed screws in the femoral plate construct, while also breaking the allograft, some other screws, and the plate itself (Figure 10).

Figure 10: The last available imaging from this patient shows a plain radiograph of a distal femoral replacement revision with a locking plate. The screws in the locking plate had been broken off. They unscrewed, as shown in this image, with an additional nonunion implant failure.

Surgery/Phase 7

A tertiary center performed a total femoral arthroplasty. At that time, the patient had stopped following up on the original arthroplasty and was subsequently lost to follow-up, and the outcomes were unknown.

Discussion/Summary

This case involves a rare pathology and an unusual patient. The fractures, including one of a non-modular femoral component, have never been reported before. These failures likely stem from two main factors: an unrecognized internal rotation of the distal tibia and excessive, intense cycling. Without both, the fractures wouldn’t have occurred. The initial osteotomy resulted in a 60-degree internal tibial rotation; however, the tibial component alignment remained unaffected, ensuring good intraoperative alignment. However, correcting the internal rotation was impossible due to ligament balancing. The rotation alone wouldn’t have caused such fractures without the combination of clipless pedals and high-intensity cycling. The patient used specialized gear for competitive cycling, which locked the left foot parallel to the bike frame, forcing external knee rotation and misaligning the mechanical axis. The patient performed 4800-6600 revolutions per hour, generating up to twice their body weight in force during standing. These forces, combined with the lack of foot rotation allowed by the clipless pedals, led to repeated implant failures, a scenario not previously reported. The pedals didn’t allow for the natural internal and external rotations needed for proper knee kinematics. When cyclists stand and pull down on the handles to increase pedal force, the force can be equivalent to or even double their body weight. The unexpected rotational forces led to repeated implant failures in this patient, which have not been reported before. The clipless pedal prevented foot rotation during knee flexion and extension, disrupting the natural internal and external rotations needed for proper knee kinematics (“screw home” mechanism). While modern clipless pedals allow 10-15 degrees of “float,” this patient’s Pedal allowed no float, as it was not designed for it.

Conclusion

This case highlights the potential risks of clipless pedals for TKA patients. While cycling is typically safe for most, individuals with uncorrected rotational deformities may experience unique implant failures due to repetitive, constrained foot movements, such as cycling or rowing. Surgeons should assess for malrotation in TKA patients with a history of prior fractures, and if present, advise against high-intensity, repetitive exercises that involve constrained foot movements.

References

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