A biomechanical analysis of a cadaveric model of early weight bearing in unstable ankle fractures following open reduction and internal fixation (ORIF) found no occurrence of significant fracture displacement, hardware failure, or new fracture.

The authors of the study, presented by Eric W. Tan, MD, of the University of Southern California, at the 2016 Specialty Day Meeting of the American Orthopaedic Foot & Ankle Society in Orlando, Fla., hypothesized that axial loading of surgically treated bimalleolar and trimalleolar ankle fractures would result in fracture displacement of less than 1 mm. In addition to evaluating the amount of displacement that occurred in the early loading phase, they sought to determine whether any correlation of displacement to bone mineral density (BMD) could be detected.

"In most cases there are pretty clear guidelines for who needs treatment for unstable ankle fractures and what kind of treatment they need," Dr. Tan said. "However, there is no consensus regarding the ideal postoperative protocol following surgical fixation of these fractures." In most cases, he noted, the fractures are splinted in the operating room and then patients are not weight bearing for 4 to 6 weeks. "That will vary from provider to provider. As with most things in the orthopaedic literature, the principles may not be based on established data. In most cases it's done because of convention or based on personal observation or expert opinion."

Previous basic science and animal studies have demonstrated that mechanical loading in the early postoperative period encourages bone healing, Dr. Tan said. "In addition, the rate and timing of loading appear to be important." Previous clinical studies have been done, but are limited in scope. "Immobilization has been associated with benefits such as lowering incidence of wound complications, but may result in increased stiffness or increased risk of complications such as deep vein thrombosis.

"On the other hand," he continued, "early weight-bearing mobilization in some studies has been shown to have a benefit, with a quicker return to work and increased short-term improvements in range of motion and function. Some studies have shown no difference in regard to wound complications or loss of reduction. Basic science and clinical studies suggest that early weight bearing appears to be as safe as immobilization but does confer some possible additional benefits."

Weight bearing to the test
In this study, biomechanical testing was performed on 24 fresh-frozen lower extremities. Specimens were divided into the following groups:

• Group 1 (n = 6): bimalleolar fracture
• Group 2 (n = 9): trimalleolar fracture + unfixed, small posterior fracture (15–20 percent).
• Group 3 (n = 9): trimalleolar fracture + fixed, large posterior fracture (≥ 33 percent)

"We chose these groups because they represent the most severe and unstable ankle fractures," Dr. Tan said.

Ankle fractures were created using custom-designed guides that allowed for reproducible osteotomies of the medial, lateral, and posterior malleoli to create a supination external rotation IV injury model with fractures to the distal fibula, posterior malleolus (without rupture of the posterior inferior tibiofibular ligament), and medial malleolus (without deltoid ligament rupture). The syndesmosis was left intact. The angle of the osteotomy was determined from previous clinical studies that have characterized the fractures as well as biomechanical models that have been previously developed and utilized. Fixation was performed using standard orthopaedic techniques.

Each specimen was subjected to axial compressive loading at a rate of 3 Hz from 0 to 1,000 N (more than 1 times body weight) for 250,000 cycles to simulate 5 weeks of full weight bearing.

"We considered failure to be fracture displacement of greater than 2 mm," Dr. Tan said. "In orthopaedics, most fractures with up to 2 mm of displacement usually can be treated nonsurgically. However, if the fracture is more than 2 mm displaced, surgical repair is often necessary."

The average fracture displacement in all cases was significantly less than 1 mm (P < 0.05), with all individual measurements less than 0.6 mm. In addition there were no cases of any hardware failure, nor development of any iatrogenic fractures. Focusing on the timing of when the displacement occurred, greater than 60 percent of displacement occurred within the first 50,000 cycles.

The investigators also examined the relationship, if any, between fracture displacement and BMD within the samples. Pearson correlation tests were used and suggested no relationship between BMD and the amount of fracture displacement.

The authors noted that few studies compare early versus delayed weight bearing after fixation of ankle fracture. Available results indicate that early weight bearing after internal fixation of ankle fractures may possibly lead to improved ankle range of motion and functional scores, with no difference in wound complications, or loss of reduction. "However," they wrote, "there is great heterogeneity within the studies with regard to the preoperative ankle fracture patterns included, in addition to the postoperative immobilization, motion, and weight-bearing protocols utilized."

In the study Dr. Tan presented, most of the total fracture displacement that was observed was seen within the first 50,000 cycles of loading. The authors noted: "This suggests that most displacement occurred at the initiation of simulated weight bearing and reached a plateau as loading continued. It is possible that this finding represents settling of the fracture, which could be associated with benefit clinically. However, no conclusions can be drawn from this finding based on the current study, and further study is warranted."

Among the limitations of the study, Dr. Tan noted, are that it is a cadaveric study, and the biomechanical fracture simulation "prevents a true anatomic reduction of the fracture and limits additional bony stabilization that can occur through the interdigitation of the fracture surfaces." Further, the reproducible osteotomies that were performed resulted in fractures without any comminution. "Ligamentous injuries were not performed or reproduced," he said.

Another potential limitation, Dr. Tan noted, is the amount of loading—1,000 N—used for the study. That force is at least 1 times body weight, but less than the 2 to 4 times body weight that the ankle may bear during ambulation. "Therefore, the results of this study should be accepted as a best-case scenario that does not account for any out-of-plane loading or muscle loading that may be present in unrestricted ambulation," the authors wrote. "Future biomechanical studies should be performed, accounting for multiplanar axial loading."

Clinical study also supports weight bearing
A few days earlier at the 2016 AAOS Annual Meeting, a retrospective study was reported showing that for patients who underwent ORIF for an ankle fracture, weight bearing as tolerated was safe, regardless of the fracture pattern.

The study, presented by Fernando A. Peña, MD, of the University of Minnesota, compared functional results and level of complications seen with immediate weight bearing as tolerated (WBAT) after ORIF in 159 patients versus 222 patients treated with ORIF and a traditional postoperative protocol of non-weight bearing (NWB) for 6 weeks after surgery. Those in the WBAT group had an average time to weight bearing of 2.6 weeks, and at 6 weeks they showed a higher level of function compared to the NWB group (P = 0.04); no significant differences in bother or functional indices were seen at all other time points.

Dr. Peña said that the findings confirm the researchers' hypothesis that patients could safely bear weight shortly after surgery, with no more adverse consequences than those who kept weight off the foot for the traditional period. The hypothesis emerged from clinical experience and patient behavior; his team undertook the study "to find out if patients were right—some of them were walking after surgery against medical advice."

The results demonstrate, Dr. Peña said, that "patients with normal sensation on their ankles/feet are perfectly safe walking after surgery for an ankle fracture."

Coauthors with Dr. Tan are Norachart Sirisreetreerux, MD; Adrian G. Paez, BS; Brent G. Parks, MSc; Paul G. Talusan, MD; Lew C. Schon, MD; and Erik A. Hasenboehler, MD. Dr. Peña's coauthors of "Weight Bearing After Open Reduction and Internal Fixation of Ankle Fractures" are Jordan Hauschild, MD; Robby Sikka, MD; and Megan Reams, MA/OTR/L.

The authors' disclosure information can be accessed at www.aaos.org/disclosure

Terry Stanton is the senior science writer for AAOS Now. He can be reached at tstanton@aaos.org

Bottom Line

• No consensus exists regarding the timing of weight bearing after surgical fixation of unstable ankle fractures.
• The biomechanical study simulated 5 weeks of full weight bearing on cadaver lower extremities with one of three types of ankle fracture.
• Average displacement at all fracture sites was significantly less than 1 mm, with no hardware failure or new fractures. No correlation was seen between fracture displacement and BMD.
• The study supports further investigation of early weight-bearing postoperative protocols after fixation of unstable ankle fractures