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The fixation of comminuted distal radius fractures using wrist-spanning dorsal bridge plates has been shown to have good postoperative results. We hypothesized that using a stiffer bridge plate construct results in less fracture deformation with loads required for immediate crutch weight bearing.
We created a comminuted, extra-articular fracture in 7 cadaveric radii, which were fixed using dorsal bridge plates. The specimens were positioned to simulate crutch/walker weight bearing and axially loaded to failure. The axial load and mode of failure were measured using 2- and 5-mm osteotomy deformations as cutoffs. Bearing 50% and 22% of the body weight was representative of the force transmitted through crutch and walker weight bearing, respectively.
The load to failure at 2-mm deformation was greater than 22% body weight for 2 of 7 specimens and greater than 50% for 1 of 7 specimens. The load to failure at 5-mm deformation was greater than 22% body weight for 6 of 7 specimens and greater than 50% for 4 of 7 specimens. The mean load to failure at 2-mm gap deformation was significantly lower than 50% body weight (110.4 N vs 339.2 N). The mean load to failure at 5-mm deformation was significantly greater than 22% body weight (351.8 N vs 149.2 N). All constructs ultimately failed through plate bending.
All constructs failed by plate bending at forces not significantly greater than the 50% body weight force required for full crutch weight bearing. The bridge plates supported forces significantly greater than the 22% body weight required for walker weight bearing 6 of 7 times when 5 mm of deformation was used as the failure cutoff.
Elderly, walker-dependent patients may be able to use their walker as tolerated immediately after dorsal bridge plate fixation for extra-articular fractures. However, patients should not be allowed to bear full weight using crutches immediately after bridge plating.
Elderly patients and patients with polytrauma with concomitant lower-extremity injuries are notable populations because they often rely on upper-extremity weight bearing for ambulation. Tinsley and Ilyas
have previously referred to these patients as “functional quadrupeds;” their dependence on assistive devices, such as a walker or crutches, means that they require all 4 limbs to mobilize. Severely comminuted distal radius fractures pose particular management challenges in these patients because traditional strategies, such as volar plating, fragment-specific plate fixation, and closed reduction with pinning and casting, do not allow for early weight bearing. External fixation can be used to allow early weight bearing, but it carries a high risk of complications.
Like external fixation, spanning bridge plates cross the radiocarpal joint and rely on distraction and ligamentotaxis to achieve and maintain fracture reduction. To accomplish this, a plate is secured using cortical screws to the dorsal surface of the radius and second or third metacarpal through proximal and distal incisions. The plate remains implanted until the fracture has sufficiently healed, after which it is removed and the wrist’s range of motion is reinitiated. Several studies have demonstrated successful fracture healing and minimal complications using this technique.
However, the patients in these studies used platform crutches, with weight bearing through the forearm and elbow, immediately after undergoing surgical fixation; weight bearing through the wrist was not allowed until several weeks after the surgery.
Recent studies have proposed allowing immediate crutch weight bearing after bridge plate fixation to further aid in the mobilization of patients with concomitant lower-extremity injuries.
previously tested the strength of 2.4-mm distal radius-spanning bridge plates and found that these plates fail at forces below those expected in crutch weight bearing. However, the plates used in their study had screw holes in the middle, and they used screws smaller than those commonly used at our center. The purpose of this study was to assess the axial load to failure of a 2.7- or 3.2-mm plate in a cadaveric immediate crutch weight-bearing model. We hypothesized that this larger bridge plate allows less fracture deformation with immediate crutch weight bearing at physiologic loads.
Materials and Methods
A total of 7 fresh, frozen cadaveric forearms with attached hands were tested (mean age 73.5 years; 5 from men and 2 from women). All the specimens were examined for external and internal defects using visual inspection and fluoroscopy, respectively, prior to testing. The specimens were maintained in a freezer at −20 °C until approximately 24 hours prior to mechanical testing; then, they were thawed to room temperature, and proximal soft tissues were removed by dissection. Institutional review board approval was not required in this cadaveric study, and all research was carried out in accordance with our institution’s ethical guidelines.
A dorsal approach to the wrist was used for each specimen to expose the dorsal distal radius. A 3-dimensional printed osteotomy cutting guide (Trimed, Inc) was positioned so that the distal cutting slit was located 10 mm proximal to the radius’ lunate facet surface (Fig. 1). This allowed a 10-mm rectangular osteotomy cut to be positioned proximal to the distal radioulnar joint. Using the cutting guide, a sagittal saw was used to start the osteotomy through the dorsal cortical surface. The guide was then removed, and a dorsal bridge plate (Trimed, Inc) was fixed to each specimen in accordance with the surgical technique guide.
The plate was secured to the second metacarpal using one 2.7-mm nonlocking screw and two 2.7-mm locking screws. It was secured proximally to the radius using one 3.2-mm nonlocking screw and two 3.2-mm locking screws. Three bicortical screws were used both distally and proximally because this number has been validated as adequate for fixation, with no significant difference from 4 screws used on each side.
Proper plate position was confirmed using fluoroscopy, and the osteotomy cut was then completed using the sagittal saw (Fig. 2). The proximal forearm soft tissue was then stripped, and the specimen was embedded in urethane (Smooth On) with the assistance of a custom alignment fixture. The distal hand was secured to a 25.4-mm-diameter wooden dowel using 3 common drywall screws through the first, second, and fifth proximal phalanges in order to simulate a hand gripping a crutch handle. Infrared-emitting diode markers were implanted into the radius distal and proximal to the osteotomy site in order to measure “fracture” displacement by tracking their movement using the Optotrak Certus system (Northern Digital Inc). The specimens were then mounted on a biaxial servohydraulic load frame (Instron Corp) using a constrained fixture that allowed motion only in an axial plane. The dowel was positioned in a vise, without the hand touching the baseplate, in order to most accurately simulate crutch weight bearing (Fig. 3).
The specimens were then axially loaded to failure at a rate of 1 mm/second, and the peak axial force at 2 and 5 mm of osteotomy displacement was recorded. Displacement was characterized by the cut osteotomy edges becoming more approximated to one another. Ultimate failure was defined as >5 mm of displacement at the osteotomy site or plate breakage/screw pull out. The method of specimen failure was also observed and recorded based on plate bending, plate breakage, or screw pull out. We defined forces ≥22% weight bearing as being required for walker ambulation and ≥50% weight bearing being required for full crutch weight bearing.
The body weight percentages were calculated based on each cadaver specimen’s recorded postmortem weight (mean 69.1 ± 14.9 kg). Generalized estimating equations were used to compare the load to failure for a 2 mm gap and 5 mm gap to the loads on the wrist required for walker and/or crutch walking (i.e. 22% and 50% of body weight, respectively). The maximum likelihood estimators of the model were adjusted for any model misspecification using classic sandwich estimation. Pairwise comparisons between the actual load to failure for each specimen and loads for 22% and 50% of the body weight were performed using orthogonal contrasts. The Holm test was used to calculate adjusted P values and confidence intervals. Statistical significance was established at P < .05, and all interval estimates were calculated for 95% confidence.
Power and sample size calculation
The sample size was estimated with an intent to detect a 1.5-SD difference between the recorded loads to failure and the 50% and 22% weight-bearing load-to-failure values while accommodating the Bonferroni adjusted per comparison (P < .025) necessary to maintain an overall α of 0.05 across the 2 hypotheses for each gap definition that we intended to test. The Bonferroni adjustment was chosen for the purpose of sample size estimation because it is highly conservative. The Holm test, which was used in all analyses, is based on the empirical P value attained at the time of data analysis, which was unavailable a priori. Given these parameters, we estimated that a sample size of 7 specimens would have a power of approximately 85.2%.
Complete load-to-failure data were available for 7 specimens for the 2-mm gap displacement and 6 specimens for the 5-mm gap displacement. The seventh specimen never reached a 5-mm gap displacement because the plate bent, only reaching 4.3 mm. A sensitivity analysis was performed in which the missing value for the seventh specimen was replaced with the load-to-failure value for the 4.3-mm gap displacement, and the findings were comparable. Since this specimen never reached the 5-mm displacement target and the sensitivity analysis showed comparable results, this data point was excluded from the mean calculation for each group. The load to failure for the 2-mm fracture gap displacement was greater than 22% of the body weight for 2 of the 7 specimens and greater than 50% of the body weight for 1 of the 7 specimens. The load to failure for a 5-mm fracture gap displacement was greater than 22% of the body weight for 6 of the 7 specimens and greater than 50% of the body weight for 4 of the 7 specimens. The mean load to failure for the 2-mm gap was 110.4 N, which was significantly lower than the load required for 50% of weight bearing, 339.2 N (P < .01). There was no statistically significant difference in the mean load to failure for the 2-mm gap and the mean load for 22% of weight bearing (110.4 N vs 149.2 N, P = .30) (Fig. 4). The mean load to failure for the 5-mm gap was 351.8 N, which was significantly greater than the load required for 22% of weight bearing, 149.2 N (P = .03). There was no statistically significant difference in the mean load to failure for the 5-mm gap displacement and the load for 50% of weight bearing (351.8 N vs 339.2 N, P = .87) (Fig. 5). The mean initial stiffness of the constructs was 20.75 ± 9.38 N/mm. All the constructs failed because of plate bending during flexion and pronation (Fig. 6). No constructs failed because of screw pull out or fracture.
The fixation of highly comminuted distal radius fractures using wrist-spanning dorsal bridge plates has been shown to have good postoperative results and allow for early platform weight bearing.
Some authors have advocated allowing immediate full weight bearing after dorsal bridge plate placement, but a prior biomechanical study showed that the plates fail at the loads required for crutch weight bearing.
Because in that study, a bridge plate with screw holes in the center of the plate was used and the plate was fixed using 2.4-mm screws, we hypothesized that using a solid bridge plate construct and larger screws results in less fracture deformation with loads required for immediate crutch weight bearing. The solid plate has no holes in the center and is made of stainless steel; therefore, it has a stiffer elastic modulus than other titanium options. Slavens and Harris
previously determined that each upper extremity sees forces equivalent to 22%–50% of body weight during ambulation using crutches. Based on these established biomechanical standards, we chose 50% body weight as our surrogate for full crutch weight bearing in a patient who has a lower-extremity injury because that is the maximum force the bridge plate construct will have to withstand in those circumstances. We surmise that 22% of the body weight is the force experienced when the patient’s uninjured foot is on the ground and the crutches are only used for balance in a tripod position. We assume that the force in this phase of crutch ambulation is similar to the force experienced when ambulating using a walker without a lower-extremity injury because 1 foot is always in contact with the ground during walking. Establishing these values makes our study relatable to 2 indications for dorsal bridge plate use: high-energy polytrauma and comminuted fractures in elderly patients dependent on assistive devices for ambulation.
Less than 2 mm of deformity has been described as acceptable for intra-articular stepoffs and postoperative reductions, whereas less than 5 mm of shortening has been deemed acceptable for extra-articular fractures, especially in the elderly.
Because of the difference in acceptable distal radius reductions based on fracture pattern, we chose to use 2 different definitions of failure, 2 and 5 mm of fracture deformation, to test our hypothesis. The plates withstood walker weight bearing (22% of body weight) in only 2 of the 7 specimens when the 2-mm osteotomy deformation was used as the cutoff for failure, and they supported full crutch weight bearing (50% of body weight) in only 1 of the 7 specimens using this cutoff. Our ultimate load-to-failure results at this cutoff were consistent with those of the study by Huang et al,
wherein the dorsal bridge plates supported 332 ± 138 N at 2 mm of deformation, which was less than the 350–400 N that they had determined as 50% of the body weight for an 80-kg individual. Our findings were also consistent with those of a study by Alluri et al,
who found a mean load to failure of 152.7 ± 50.2 N at the 2-mm gap deformity for dorsal bridge plates fixed in the second metacarpal, although those specimens were loaded in wrist flexion as opposed to those loaded in axial compression. The plates supported walker weight bearing (22% of body weight) in 6 of the 7 specimens when the 5-mm deformation was used as the definition of failure, but only 4 of the 7 specimens supported full crutch weight bearing (50% of body weight).
Although it is expected that accepting more deformation allows for higher loads before failure, 5 mm of deformation may be a more realistic radiographic parameter to be considered acceptable in elderly patients, given the multiple studies showing little correlation between radiographic malunion and functional outcome.
Only 4 of 7 of our dorsal bridge plate constructs could resist forces significantly greater than the 50% of the body weight required for full crutch weight bearing when a 5-mm fracture gap deformity was used as a failure cutoff. However, they did withstand forces significantly greater than 22% of weight bearing estimated for walker weight bearing in 6 of 7 specimens at this failure cutoff. This finding is important because of its implications for postoperative weight-bearing protocols. Platform weight bearing can be difficult in patients who require a walker for ambulation at baseline.
Based on our assumption that 22% of weight bearing is required for walker ambulation in patients without lower-extremity injuries, our findings suggest that some walker-dependent patients may be able to use their operative extremity, as tolerated, for assisted ambulation after dorsal bridge plate fixation.
Furthermore, we believe that our measured loads to failure were likely minimal forces that these plates could withstand invivo. The measured forces at the 2- and 5-mm gap displacement were likely limited by the block osteotomy design of our testing, which required all forces in the radius to be directed through the plate. In a clinical scenario, there would be bony contact, even in the most comminuted fractures, which would help distribute the force and may allow higher loads prior to failure. This potential difference was exemplified by the lower initial plate stiffness of our constructs (20.8 ± 9.4 N/mm) compared with that measured for either the volar locking plate or dorsal bridge plate constructs in the study by Huang et al
(51.4 ± 26.0 N/mm vs 32.4 ± 17.2 N/mm, respectively). Based on the characteristics of the plates used in each trial, we expected our construct’s stiffness to be higher. However, the block osteotomy we employed has less inherent stability than the dorsal wedge osteotomy used in the testing by Huang et al,
which may have accounted for the observed differences. We believe that block osteotomy more closely represents comminuted fractures in which these plates are typically used, and this aspect of our study design further supports our data suggesting that wrist fractures with dorsal-spanning plates can withstand forces generated due to weight bearing using a walker.
A limitation of our study was the large effect size used when calculating our a priori power analysis. We chose a detectable difference of 1.5 SD to estimate the effect size based on prior literature, but there is a concern that the study may be underpowered to detect smaller, possibly relevant differences between the loads at failure and cutoff body weight percentages.
Based on the calculations in the current study, 7 specimens were needed to detect a 1.5-SD difference at a power of 85.2%. Increasing the number of specimens to 10 only decreases the detectable difference to 1.32 SD. We do not think that this small improvement justifies the considerably increased costs associated with more materials. We do not think that our results would have changed meaningfully with those additional specimens.
In summary, we found that most fractures treated with a solid, stainless steel bridge plate acceptably resist deformation from the forces required for walker weight bearing in the case of extra-articular fractures (5-mm deformation failure) but not in the case of intra-articular fractures (2-mm deformation failure). Therefore, patients should not be allowed to bear full weight using crutches immediately after undergoing fixation with a bridge plate construct. However, walker-dependent patients may be able to immediately use their walker to ambulate and move as tolerated. Further biomechanical studies could evaluate the fatigue strength of these fixation constructs, and future clinical research could prospectively evaluate outcomes in walker-dependent patients who are allowed to bear weight as tolerated after undergoing dorsal bridge plate fixation.
We thank Rachel Schilkowsky and Peter Wronski for their help conducting our experiments in the biomechanics laboratory. Materials, including cadavers, plates and instrument sets, were provided by Trimed, Inc.: Santa Clarita, CA, USA. Additional funding was provided by our institutions research fund, the Rhode Island Orthopedic Foundation: Providence, RI, USA.