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Corresponding author: Frederick W. Werner, MME. Department of Orthopedic Surgery, SUNY Upstate Medical University, 750. E. Adams St., Syracuse, NY 13210.
To determine the 6 degrees of freedom forces and moments in the distal radius that occur during a pushup or other active wrist motions.
Methods
Eight fresh-frozen cadaveric wrists were moved through 6 physiological motions and held at 1 static pushup position while the force through the distal radius was measured with a 6 degrees of freedom load cell. Three levels of compressive force were applied at the pushup position.
Results
Active wrist motions caused axial forces up to 283 N and moments up to 0.7 N-m. Those motions with a smaller range had significantly smaller axial forces than the larger flexion-extension or dart-thrower’s motions. With an 89-N pushup force applied, the average maximum axial force was 69 N, the radially directed force was 12 N, and the moment about the radioulnar axis was 2.3 N-m. Linear extrapolation of the forces to 100% body weight indicate that the axial force going through the distal radius would be 663 N, the radial force would be 147 N, and the moment about the radioulnar axis would be 18.6 N-m.
Conclusions
Large distal radius forces and moments can occur during pushup and active wrist motions. There are not only large axial compressive forces but also nontrivial radially directed forces.
Clinical relevance
This study may help surgeons and therapists better treat complicated distal radius fractures as well as provide for better comparisons of existing or new distal radius plates and constructs that are designed to treat these complicated loading patterns.
to treat simple and complicated fractures. To evaluate new plate designs and constructs, researchers have typically applied moments in the sagittal plane or axial forces to constructs having a plate secured to cadaver radii or sawbones models. They suggest that one design is better if it has the greatest force to failure or stiffness. Frequently, only axial forces were applied, such as in the study by Gesensway et al
summarized 4 stages of fracture healing that occur and commented on the corresponding activity levels that are safe for patients with a healing fracture. Rehabilitation that allows gentle wrist motion may be allowed early on, depending upon the type of fracture and treatment. They note that, with time, active wrist motions are allowed and finally strengthening exercises are permitted. Little is known about the forces that occur in the distal radius during activities of daily living that a fracture fixation plate or construct might need to support during rehabilitation, an unexpected fall, or pushing up from a lying position. Greenburg et al
measured the in vitro radial and ulnar force during physiological motions but only reported them as percent force distributions between the 2 bones. Osada et al
compared the deformation and strength of different distal radius plating constructs with 100 N, 250 N, or failure forces applied. The 100-N force was selected based on theoretical calculations of the loading during active wrist motion. The 250-N force was chosen to represent physiological loading with active finger flexion.
One goal of this study was to determine the magnitude of forces that occur in the distal radius during simple wrist motions. These data can directly be used to confirm the validity of these previous estimates. These results may also clarify whether certain motions cause smaller distal radius forces and, therefore, may be preferable early on during rehabilitation. Another goal of this study was to determine the direction (eg, proximal, dorsal, radial) of the forces and moments in the distal radius during unrestricted wrist motions and during a pushup. In this study, we use the term pushup to reflect the loading when a person’s wrist is maximally extended and forces are applied during various activities during the day, such as when someone lifts themselves out of a chair, moves furniture to vacuum beneath it, or even reaches up to put a heavy box onto a high shelf. For some patients seeking to return to competitive sports, physical exercise pushups may be included toward the end of a rehabilitation protocol.
The activities examined in this study represent a range from lightly loaded passive activities to higher load activities with the wrist extended that might occur during the healing process. The direction of loading during these activities may have an impact on the timing of when they are allowed for different types of fractures because a plate or construct may have to continue to support these forces while the healing process is completed. A third goal of this study was to aid in the evaluation of new plate designs. Simply designing a stronger plate may not be beneficial. To reduce soft tissue impingement, a thinner plate may be desirable if it can support the forces during healing of a fracture. Therefore, information on these forces may help determine ideal loading profiles to apply when testing plates and constructs and in the treatment of complicated fractures.
The purpose of this study was to determine the 6 degrees of freedom forces and moments in the distal radius that occur during a pushup or other active wrist motions.
Methods
Eight fresh-frozen cadaveric wrists (average age, 79 years [range, 54–92 years], 6 men) were tested in a computer-controlled, servohydraulic simulator.
The sample size of 8 wrists was based on previous studies using similar methods but for different clinical applications. In 1 study, 7 wrists were used,
Using the simulator, each wrist was dynamically moved through simulated active arcs of 2 flexion-extension, 1 radioulnar deviation, and 3 dart-thrower’s motions. The small flexion-extension motion was from 30° extension to 30° of flexion. The large flexion-extension motion was from 30° extension to 50° flexion. The radioulnar deviation motion was from 20° of ulnar deviation to 10° of radial deviation. The small dart-thrower’s motion had a range from 30° of flexion and 10° of ulnar deviation to 30° of extension and 10° of radial deviation. The larger dart-thrower’s motion had the same extension and radial deviation but had an increase to 50° of flexion at 10° of ulnar deviation. The third dart-thrower’s motion was similar to the larger dart-thrower’s motion, but all of its motion was offset ulnarly 5° to better represent the dart-thrower’s motion seen in many daily activities, based on a study by Garg et al.
In that study, they reported that a number of activities of daily living occur with a dart-thrower’s motion with the path of motion ulnarly offset from neutral. These 6 motions were chosen to reflect what is considered to be a functional wrist range of motion and to include several dart-thrower’s motions, the importance of which has been described by Moritomo et al.
These motions were caused by pulling on 5 wrist tendons—the extensor carpi ulnaris, the extensor carpi radialis brevis, the extensor carpi radialis longus, the flexor carpi radialis, and the flexor carpi ulnaris—with 5 different hydraulic actuators. Each motion was controlled by an algorithm that uses agonistic tendon loading to cause the motion and antagonistic tendon loading to resist the motion. Out-of-plane motions such as radial deviation during a flexion-extension motion were corrected every 12.2 milliseconds with an algorithm that caused both ulnar deviators to be agonists until the motion reached neutral radioulnar deviation. Each wrist was moved through 6 cycles of each type of wrist motion. For each motion, the data from the fourth cycle of motion was used to allow for soft tissue preconditioning during the first 3 cycles.
Three levels of nominal antagonistic forces were used in each motion (8.9 N, 13.5, and 20 N per tendon) to represent increasing resistive forces in different activities. These forces were not necessarily constant because if, during a wrist motion, an antagonistic tendon was required to apply a corrective force and thus became an agonist, the force in that tendon might exceed its otherwise specified antagonistic force.
Forces were measured with a 6 degrees of freedom load cell attached to the distal radius (Figs. 1, 2). This load cell measured 3 orthogonal forces (axial compression, dorsal-volar, and radioulnar) and 3 orthogonal moments about these axes. Because these measurements are orthogonal to each other, these 6 forces and moments (corresponding to 6 degrees of freedom) completely define the force transmitted through a rigid body or bone. After attaching each end of the load cell to the distal radius, a segment of bone between the 2 attachment sites was removed (Fig. 1). This ensured that the forces distal to the load cell passed through the load cell and could be measured. Furthermore, the soft tissues between the 2 attachment sites of the load cell were sectioned to have all forces pass through the load cell. Because the recorded load cell data were relative to the origin of the load cell, the load cell’s location relative to the distal radius was determined. The measured 3 orthogonal forces and 3 moments were then mathematically transformed to a location in the distal radius 3 cm proximal to the junction of the radiolunate and radioscaphoid fossae. This location was chosen to reflect a site at which distal radius fractures commonly occur.
Figure 1Cadaver wrist and forearm positioned in the wrist simulator. A 6 degrees of freedom load cell was attached to the radius. An osteotomy was performed between the 2 attachments of the load cell to ensure all force was transmitted through the load cell. A positive radially directed force is shown as a force acting on the distal radius relative to the proximal radius.
Figure 2In this radial view of a wrist and forearm in the simulator, a positive dorsally directed force is shown as a force acting on the distal radius relative to the proximal radius.
Forces in the distal radius were defined to be positive in a proximal direction (corresponding to axial loading with compression being positive), in a radial direction, and in a dorsal direction of the distal radius relative to the midshaft radius. Moments about these axes were also determined. A positive moment about the radioulnar axis is a moment that could cause dorsal angulation of a distal radius fracture.
The last 7 of the 8 wrists were also tested at an extended wrist pushup position (average, 70°) while 17.8, 44.5, and 89 N compressive forces were applied to the wrist (Fig. 3). The maximal extension angle was determined by applying a slight extension force to each wrist, positioning the loading plate at that angle, and then applying the applied compressive force. The compressive force was varied by using a turnbuckle that was in series with an axial load cell. No wrist tendon forces were applied at the pushup position. The results from the 3 applied compressive forces for each wrist were linearly extrapolated to 100% body weight for each corresponding specimen’s body weight.
Figure 3Wrist positioned in a pushup position. Increasing pushup forces can be applied by a turnbuckle that is measured by an inline axial load cell.
The peak force and moment during the fourth cycle of each motion was determined. Differences between the peak forces and moments for the 6 wrist motions were compared statistically using a 1-way repeated measures analysis of variance with a Bonferroni correction for multiple comparisons. This comparison of the 6 wrist motions was analyzed for 8 arms at the intermediate antagonistic force level (13.5 N/tendon) because, in the first 2 of the 8 tested arms, during 1 dart-thrower’s motion, data were not acquired at the largest antagonistic force level (20 N/tendon).
Results
Active wrist motions caused axial forces up to 283 N (Table 1) and moments up to 0.7 N-m when the largest antagonistic force (20 N) was used during the motion. Whereas nontrivial (11–13 N) radially directed forces occurred during all motions, there were minimal (1 N) dorsally or volarly directed forces. In comparing the different wrist motions, those with a smaller range of motion, specifically the small flexion-extension, the radioulnar deviation, and the small dart-thrower’s motions had significantly (P < .05) smaller axial forces than the larger flexion-extension or the 2 larger dart-thrower’s motions. There was no significant difference in the radially directed forces among the 6 wrist motions (P > .98).
Each increase in the nominal antagonistic tendon force caused a significant (P < .05) increase in the axial force during the large flexion-extension, radioulnar deviation, small dart-thrower’s and large dart-thrower’s motions. For the small flexion-extension and the dart-thrower’s motion with an ulnar offset, the axial force was significantly larger with the 20-N antagonistic force compared with the 8.9-N force. With a 13.5-N nominal antagonistic force level, the average peak axial force increase over the 6 motions was 20.7% compared with the 8.9-N antagonistic force. With a 20-N antagonistic force level, the axial force increase was 50% over the 8.9-N level.
At the highest applied pushup force (89 N) applied to the wrist, the average maximum axial force was 69 N, the average radially directed force was 16 N, the average dorsally directed force was 12 N, and the average moment about a radioulnar axis was 2.3 N-m. Linear extrapolation of these forces and moments for the 3 levels of applied pushup force to 100% body weight for each specimen indicate that these forces and moments would be 663 N, 147 N, 126 N, and 18.6 N-m, respectively.
Discussion
This study demonstrated that large compressive and out-of-plane forces can occur during physiological wrist motions and during a wrist pushup. The large axial forces measured during wrist motion were only due to the wrist tendon forces required to cause wrist motion. Greater forces would be expected during an activity of daily living in which resistive forces such as turning a door knob or using a tool might occur. The radially directed forces were most likely related to the axial force being supported in part by the radioscaphoid fossa of the distal radius.
This study also demonstrated that reduced axial loading occurs during the smaller wrist motions. These motions may be preferable during patient rehabilitation following distal radius fracture. In contrast to the observations that a dart-thrower’s motion may be of benefit following scapholunate interosseous ligament injury (due to reduced scaphoid and lunate motion during a dart-thrower’s motion
), a dart-thrower’s motion does not appear to decrease the forces at the level at which many distal radius fractures occur.
Our results demonstrate that loading across the wrist can occur in different directions and magnitudes depending upon the wrist motion or activity. For example, large distal radius moments can occur during a pushup. This may help explain why dorsal angulation of the distal radius fragment usually occurs after a fall with an outstretched hand. In general, these observed out-of-plane forces and moments need to be supported by a plate or construct. The plate’s design and placement may affect how well it can support different types of loading. It may well be that a dorsally placed plate may provide a buttress to a dorsally displaced fragment. A plate’s design and location may make it better suited to support different kinds of loading. It may also be that how long certain activities, such as a pushup, should be postponed after the acute postfracture stage may depend upon the plate design and placement.
Most biomechanical studies that compare different fracture fixation methods apply sagittal plane bending moments or axial loads to the construct.
This study demonstrates that the loading in the distal radius is a combination of axial forces, radially directed forces, and moments. Biomechanical studies that test plates or plate constructs should use a combination of moments and loads that better reflect the in vivo loading to which the radius is exposed. Based on our findings, we suggest that, at minimum, studies examining fracture fixation techniques in a cadaver model should apply a minimum of 300 N of axial force and a 15-N radially directed force under cyclic loading. Greater forces should be used in studies that look at fracture nonunion and when more strenuous activities might occur earlier during the rehabilitation period.
showed that, during forearm rotation, the force through the distal radius was approximately 70 N while the wrist was kept in a neutral position. In vitro transverse distal radioulnar joint forces measured by Greenberg et al
during 3 similar wrist motions were on the same magnitude as we measured; however, their data were not transformed to 3 cm proximal to the distal radius and they sectioned additional soft tissues.
Some surgeons are advocating for an accelerated rehabilitation protocol in patients that undergo open reduction internal fixation for distal radius fractures. In a prospective, randomized controlled study, Brehmer and Husband
Accelerated rehabilitation compared with a standard protocol after distal radius fractures treated with volar open reduction and internal fixation: a prospective, randomized, controlled study.
found consistently lower Disabilities of the Arm, Shoulder, and Hand scores and improved range of motion in postoperative patients who began stretching exercises 2 weeks after surgery (compared with the standard 6 weeks). These protocols were extrapolated from forces acting on the distal radius through a pinching motion. Force data from our study could be similarly applied to rehabilitation protocols for postoperative patients. Krischak et al
suggested a home exercise program for postoperative distal radius fracture patients involving active motion in both the radioulnar deviation and the sagittal planes starting 2 weeks after surgery. Our data indicate larger force moments occur with larger motion arcs, suggesting that limits should be set for the range of motion parameters used in therapy.
There are several limitations to this study. First, these forces were measured in fresh cadaver specimens, which may only approximate what occurs in vivo. During testing, each forearm was positioned in a vertical position. If the forearm was placed in a different position, different wrist tendon forces might have been needed to cause wrist motion and different forces in the distal radius might have been calculated. The average pushup extension position used in this study was 70°. Some specimens could not reach greater extension angles without applying greater forces than what we wanted to apply in the testing setup. A further limitation is that we did not apply any wrist tendon loading during the simulated pushup. In this study, the force data were transformed to a point 3 cm proximal to the radial articulating surface and thus only apply to fracture fixation considerations in that region of the distal radius.
The force and moment data acquired in this study should help surgeons be aware of nonplanar loading that can occur during wrist motion. The results emphasize the occurrence of distal radius axial loading and that activities that require large wrist tendon forces across the wrist will cause larger axial forces, which would need to be supported by a plate construct. The results also support the importance of having bone-to-bone contact during fracture fixation such that the plate construct is not supporting all of the axial force through the distal radius. These data can provide a basis for suggested loading profiles that could be used in isolated testing of fracture fixation devices or when tested as part of a bone/screw/plate construct. Having a better understanding of the forces that a plate construct needs to support may allow for better designed plates. These force and moment results may also be of use when considering rehabilitation protocols well after the immediate postoperative therapy stage or when examining other wrist injuries and treatments of the distal radius.
Acknowledgment
Funded by the Department of Orthopedic Surgery, SUNY Upstate Medical University, Syracuse, NY.
Accelerated rehabilitation compared with a standard protocol after distal radius fractures treated with volar open reduction and internal fixation: a prospective, randomized, controlled study.
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