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
This post comes from Fred Nelson, MD, an orthopaedic surgeon in the Department of Orthopedics at Henry Ford Hospital and a clinical associate professor at Wayne State Medical School. Some of Dr. Nelson’s tips go out weekly to more than 3,000 members of the Orthopaedic Research Society (ORS), and all are distributed to more than 30 orthopaedic residency programs. Those not sent to the ORS are periodically reposted in OrthoBuzz with the permission of Dr. Nelson.
Determining when a fracture has healed enough for functional use can be difficult. The Radiographic Union Score for Tibia fractures (RUST) assesses fracture healing on a continuous scale from 4 to 12 points. Based on an evaluation of anteroposterior and lateral radiographs, RUST accounts for callus without visible fracture line (3 points), callus with visible fracture line (2 points), or absence of any callus (1 point) for each of four cortices. The modiﬁed RUST (mRUST) score subdivides the second parameter into two categories (callus present and bridging callus), creating a score ranging from 4 to 16 points. This tool has demonstrated high intraclass correlation coefficients (ICCs). However, until now, the correlation of these scores to mechanical properties of healed bone had not been demonstrated.
Cooke et al.1 evaluated both scores against the physical properties of bone healing by using a model of closed, stabilized femur fractures in 8- to 12-week-old male mice. Control mice received a normal diet and an experimental group received a phosphate-restricted diet. The physical properties of bone healing were determined with micro-computed tomography (µCT) and torsion testing on postoperative days 14, 21, 35, and 42. There were 10 to 16 mice in each group at any given time-point.
RUST scores from five raters were determined from anteroposterior and lateral radiographic views constructed from the µCT scans. ICCs were 0.71 (mRUST) and 0.63 (RUST). Both RUST scores were positively correlated with callus bone mineral density, bone volume fraction, callus strength, and rigidity. Radiographically healed calluses with an mRUST score of ≥13 and a RUST score of ≥10 had excellent relationships to structural and biomechanical metrics.
Mechanical properties revealed the effects of delayed healing due to phosphate dietary restrictions at later time points, but no such distinctions were found in the RUST scores. Both the RUST and mRUST scores have high correlation to physical properties of bone healing, but this tool may not be reliable for detecting poor bone quality due to nutrient deficiencies.
- Cooke ME, Hussein AI, Lybrand KE, Wulff A, Simmons E, Choi JH, Litrenta J, Ricci WM, Nascone JW, O’Toole RV, Morgan EF, Gerstenfeld LC, Tornetta P 3rd. Correlation between RUST assessments of fracture healing to structural and biomechanical properties. J Orthop Res. 2018 Mar;36(3):945-953. doi: 10.1002/jor.23710. Epub 2017 Sep 20. PMID: 28833572 PMCID: PMC5823715 DOI: 10.1002/jor.23710