Mechanical factors undoubtedly play a role in the rate and quality of fracture healing. For example, the seminal work on fracture strain by the late Stephan Perren, MD helped us understand that the larger the overall fracture area, the lower the fracture strain—and that less strain encourages fracture union.
But with the variety of fracture planes and orientations, different energies imparted to produce the fracture, and multiple patient factors such as bone density, the best approaches by which to positively influence fracture-healing mechanics are still being investigated. We do know that motion mechanics come into play for nonsurgically stabilized fractures in our patients.
In the February 3, 2021 issue of The Journal, Glatt et al. provide more data on the role of micromotion in fracture healing. The authors created a 2-mm transverse tibial osteotomy in 18 goats and then used an external fixator to achieve static, rigid fixation in 6 of the osteotomized tibiae. Six other tibiae were treated with a fixator that allowed 2 mm of controlled axial micromotion for the 8-week duration of the experiment. (This so-called dynamization technique was championed more than 30 years ago by Fred Behrens, MD, who established that inducing micromotion helps stimulate maturation of fracture callus.) The remaining 6 tibiae were initially treated with dynamization, followed by rigid fixation during weeks 4 through 8—a technique known as reverse dynamization. The experimental groups simulated 3 different versions of cast or brace immobilization without surgery.
Using radiographs, micro-CT data, and torsion testing, the investigators found that, after 8 weeks, bones in the reverse-dynamization group were significantly stronger and showed more characteristics of intact, contralateral tibiae than the treated bones in the other 2 groups. I agree with the authors’ conclusion that their results “may have important consequences regarding our understanding of the optimum fixation stability necessary to maximize the regenerative capacity of bone-healing clinically.” With this experiment, Glatt et al. have added another important piece to the puzzle that Drs. Perren and Behrens started solving many years ago.
Marc Swiontkowski, MD
One key question for orthopaedic surgeons regarding revision total knee arthroplasty (TKA) is how best to affix femoral and tibial stems. The August 17, 2016 edition of the Journal of Bone & Joint Surgery contains findings from a Level I randomized trial by Heesterbeek et al . that addresses this clinical conundrum.
Thirty-two patients with Type-I or II bone defects who needed a revision TKA received the same basic implant, with the femoral components and tibial baseplates being cemented in all cases. However, in half the patients the femoral and tibial stems were cemented, and in the other half the stems were press-fit (so-called hybrid fixation).
Measuring micromotion with radiostereometric analysis (RSA) at baseline, 6 weeks, and 3, 6, 12, and 24 months, the authors found no significant between-group differences in component migration. Similarly, at the 2-year follow-up, there were no significant between-group differences in clinical scores, including KOOS and visual analog ratings of pain and satisfaction.
The authors expressed concern about what they deemed the “relatively high” number of components in both groups that migrated > 1 mm (translation) or > 1° (rotation), and they are continuing to follow all these patients to determine whether clinically relevant component loosening eventually ensues.