JBJS | The Role of the Coronoid Process in Elbow Stability : A Biomechanical Analysis of Axial Loading*

The Journal of Bone & Joint Surgery, Volume 82, Issue 12

Articles | December 01, 2000

The Role of the Coronoid Process in Elbow Stability : A Biomechanical Analysis of Axial Loading*

Robert F. Closkey, M.D.†; Joel R. Goode, M.D.†; David Kirschenbaum, M.D.‡; Ronald P. Cody, Ed.D.§

Investigation performed at the Robert Wood Johnson Medical School, New Brunswick, New Jersey
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
†Division of Orthopaedic Surgery, Robert Wood Johnson Medical School, P.O. Box 19, New Brunswick, New Jersey 08903.
‡2186 State Highway 27, Suite 1A, North Brunswick, New Jersey 08902. Please address requests for reprints to D. Kirschenbaum.
Â§Department of Environmental and Community Medicine, Robert Wood Johnson Medical School, 675 Hues Lane, Piscataway, New Jersey 08854.

The Journal of Bone & Joint Surgery. 2000; 82:1749-1749

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Abstract

Background: The current treatment of coronoid process fractures of the ulna is based on the classification system of Regan and Morrey. We found no biomechanical studies that specifically addressed the role of the coronoid process in elbow stability. In the present investigation, the elbows of cadavera were tested before and after fracture of the coronoid process to assess the stabilizing contribution of the coronoid process under axial loading.

Methods: Six fresh-frozen cadaveric elbows were tested mechanically. All soft tissue surrounding the elbow, including the skin, was left intact. An axial load compressing the elbow joint was applied along the shaft of the forearm in the sagittal plane. A displacement of fifteen millimeters per minute was applied until a load of 100 newtons was attained. Each elbow was tested in 15, 30, 45, 60, 75, 90, 105, and 120 degrees of flexion. Next, less than 25 percent, 25 to 50 percent, or more than 50 percent of the coronoid process was fractured with an osteotome under radiographic guidance, and the testing was repeated. Each elbow served as its own control, and one elbow was used for two tests; therefore, a total of seven situations were investigated. The difference in displacements between the intact and osteotomized elbows was measured.

Results: There was no significant difference, at any flexion position, in posterior axial displacement between the intact elbows and the elbows in which 50 percent or less of the coronoid process was fractured (type I and type II) (p = 0.43). There were significant differences, across all flexion positions, in posterior axial displacement between the intact elbows and the elbows in which more than 50 percent of the coronoid process was fractured (type III) (p = 0.006). Specimens with a type-III fracture also showed a significant increase in displacement compared with specimens with a type-I or type-II fracture (p = 0.012). Specifically, from 60 to 105 degrees of flexion, a significant increase in posterior translation of up to 2.4 millimeters was found (p < 0.05).

Conclusions: In response to axial load, elbows with a fracture involving more than 50 percent of the coronoid process displace more readily than elbows with a fracture involving 50 percent or less of the coronoid process, especially when the elbow is flexed 60 degrees and beyond.

Clinical Relevance: These data provide biomechanical support for the current treatment of coronoid process fractures and may help to explain why type-III coronoid process fractures often have a poor prognosis.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

Fractures of the coronoid process are uncommon injuries and are most often associated with elbow dislocation¹⁵. The goal in treating these injuries is to restore elbow motion and stability. The management of coronoid process fractures continues to evolve^6,8,12,21. Currently, treatment is based on the classification system of Regan and Morrey, who defined coronoid process fractures as type I (an avulsion fracture of the coronoid process), type II (a fracture producing a fragment of 50 percent or less of the coronoid process), or type III (a fracture with a fragment of greater than 50 percent of the coronoid process)²⁶. On the basis of the outcomes of thirty-five fractures, Cabanela and Morrey recommended early motion for stable type-I and type-II injuries and reduction and fixation for type-III fractures⁵.

Biomechanical studies of proximal ulnar fractures have been limited to olecranon injuries⁴. We found no published studies that specifically addressed the biomechanics of coronoid process fractures. The clinical literature clearly demonstrates the instability that can be associated with coronoid process fractures²².

The hypothesis of the present study was that isolated fractures of the coronoid process do not lead to subluxation or axial instability of the elbow. Rather, elbow instability results from a coronoid process injury in combination with other osseous or ligamentous injury. This hypothesis was investigated by selectively creating isolated fractures of the coronoid process in cadavera and mechanically stressing the elbow joint. The results for these injured elbows were then compared with findings that had been obtained when the elbows were intact, in order to assess the stabilizing contribution of the coronoid process during axial loading. We chose axial loading because it seems to be a common mechanism leading to a coronoid process fracture.

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Fig. 1:: Radiographs showing an unloaded elbow with a type-III fracture (left) and a loaded elbow with a type-III fracture demonstrating posterior subluxation (right).

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Fig. 2:: Graph showing the percent mean displacement, compared with that of the control elbow with an intact coronoid process, among the three types of simulated fractures. A type-I fracture involved less than 25 percent of the coronoid process; type-II, 25 to 50 percent; and type-III, more than 50 percent. The asterisks indicate that the difference between two groups - fractures involving 50 percent or less and those involving more than 50 percent of the coronoid process - was significant at p < 0.05.

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Six fresh-frozen adult cadaveric elbows were tested. The elbows were obtained from four female and two male donors who had been an average of seventy-five years old at the time of death. The elbows were thawed to room temperature and were kept moist throughout testing. All soft tissue, except for the interosseous membrane, was removed from the distal part of the forearm and the proximal part of the humerus. All soft tissue surrounding the elbow, including the skin, was left intact.

Steinmann pins were rigidly press-fit in the medullary canal of the radius and ulna so that there was no detectable motion at the interface. The pins in the forearm were held securely in a pneumatic vise set at eighty pounds per square inch (552.6 kilopascals). The vise was attached to a 100-newton load-cell on an Instron testing machine (Instron, Canton, Massachusetts). The proximal part of the humerus was fixed rigidly in a specially designed jig. The jig was bolted to the base of the Instron testing machine. The forearm was placed in the neutral position. The arm was positioned so that an axial load was applied along the shaft of the forearm in the sagittal plane.

A displacement of fifteen millimeters per minute was applied until a load of 100 newtons was attained. This rate was chosen to correspond to a mild physiological strain rate^27,32. On the basis of prior experiments^9,19,31, each intact elbow was tested in 15, 30, 45, 60, 75, 90, 105, and 120 degrees of flexion with three trials at each position. The resulting force and displacement were recorded on a flat-bed plotter. Next, less than 25 percent, 25 to 50 percent, or more than 50 percent of the coronoid process was fractured under radiographic guidance to correlate with the classification system of Regan and Morrey²⁶. A stab incision was made in the skin, and blunt dissection was carried down to the coronoid process. The collateral structures were left intact. The coronoid process was then fractured with an osteotome. Next, each injured elbow was tested in the eight different flexion positions with three trials at each position. Five elbows had a single osteotomy, and one elbow was tested after a type-II osteotomy and then again after a type-III osteotomy. A total of 312 displacement measurements were recorded. The average of the three osteotomy displacements at each flexion position was used to determine the final displacement for analysis.

Lateral radiographs of each elbow in 90 degrees of flexion after the osteotomy were reviewed to determine the percent size of the coronoid process fracture.

Statistical Methods

The primary efficacy variable was the difference, at each of the eight angles of flexion, between the displacement measurement before the procedure and that after it (the value before the procedure minus the value after it). The specimens were classified into two groups: those with a fracture involving 50 percent or less of the coronoid process (type I and type II) and those with a fracture involving more than 50 percent of the coronoid process (type III). To test for group differences with regard to the difference score, a repeated-measures analysis of variance was used, with each of the eight measurements made at the different flexion angles as the repeated-measure factor. Following this, group differences were tested at each angle with use of the Student t test. All tests were two-tailed with the level of significance set at p < 0.05. Statistical analysis was performed with use of the GLM procedure from SAS software (version 8; SAS Institute, Cary, North Carolina).

Results

Introduction | Materials and Methods | Results | Discussion | References

A total of seven coronoid process fractures were investigated: one type-I fracture involving 10 percent of the coronoid process, two type-II fractures involving 25 and 40 percent, and four type-III fractures involving 60, 65, 65, and 100 percent. The average posterior displacement in the six intact elbows was 0.7 millimeter under the 100-newton axial force. The displacement of the type-I and type-II fractures (average, 0.8 millimeter) was similar to the displacement in the intact elbows, with mean maximal displacements of 1.6 millimeters (type I) and 1.9 millimeters (type II). The maximal displacement of the type-III fractures was 2.4 millimeters. No elbows dislocated under the 100-newton axial force, but posterior subluxation was evident radiographically in the elbows with a type-III fracture (Fig. 1).

There was no significant difference, at any flexion position, in posterior axial displacement between the intact elbows and the elbows in which 50 percent or less of the coronoid process was fractured (type I and type II) (p = 0.43). There were significant differences, across all flexion positions, in posterior axial displacement between the intact elbows and the elbows in which more than 50 percent of the coronoid process was fractured (type III) (p = 0.006). Also, type-III fractures showed a significantly greater increase in displacement (average, 30 percent greater displacement compared with the control) than the type-I and type-II fractures (average, 9 percent greater displacement compared with the control) (p = 0.012). Specifically, from 60 to 105 degrees of flexion, a significant increase in posterior translation of up to 2.4 millimeters was found (p < 0.05) (Fig. 2). Finally, only the elbows with a fracture involving more than 50 percent of the coronoid process displaced more than the control elbows at all flexion angles (p = 0.006).

Within the type-III group, the displacement did not appear to be directly proportional to the amount of the coronoid process that was fractured. For example, the elbow with a complete (100 percent) fracture did not displace more than elbows with a smaller (65 percent) fracture.

As described, three trials were conducted at each flexion position and the displacements were averaged. We found a high degree of precision during testing, as demonstrated by the standard deviation of the displacements used to arrive at the average displacement for each test. The range of average displacements for the 312 tests was 0.2 to 2.4 millimeters, with a standard deviation ranging from 0.0 to 0.1 millimeter.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Morrey et al. reported that a range of elbow flexion of 30 to 130 degrees is required for most activities of daily living¹⁹. Data from experiments on cadaveric elbows have shown that, during this motion, the axis of rotation of the elbow for planar flexion-extension lies approximately at the trochlear center^14,16,18. These studies form the basis for our testing the elbow from 15 to 120 degrees of flexion and for our assumption that the motion is planar about the center of the trochlea.

Force across the elbow is a combination of the externally applied load and the internally generated muscle force. Twenty-four muscles cross the elbow, with each exerting a load on the joint, and during normal daily activity the total joint load can exceed 3000 newtons^1,3. An et al. demonstrated that the centroid of the major elbow flexors and extensors lies near the midaxis, and the majority of the force that they generate is axial³. Although large forces exist during physiological activity, we wished to determine the minimal loads that might produce instability. Eckstein et al. showed that, with as little as forty newtons at the elbow, nearly the entire articular surface of the coronoid is in contact with the trochlea, and this contact area increases with additional force^7,29. With up to 100 newtons the majority of the load in the forearm is transferred to the ulnohumeral joint, and with additional loading the force is shared between the radiohumeral and ulnohumeral joints^9,31. In our investigation, a gradual load to 100 newtons was applied axially to the forearm to determine if an injury to the coronoid process leads to elbow instability under minimal physiological loads.

Axial loading was chosen also because this is a common mechanism of a clinical coronoid process fracture²¹. Amis and Miller performed mechanical tests that demonstrated that coronoid fractures are caused by indirect force transmitted axially through the forearm². Some type-I fractures may be avulsion injuries^12,31, but Neill Cage et al. determined that there are no soft-tissue attachments to the tip of the coronoid process²³.

Forearm rotation has been shown to influence the distribution of force across the elbow²⁰. The neutral position appeared to distribute the forces most evenly. We thought that using this position would limit rotational bias of force transmission between the radius and the ulna.

The results of this experiment provide some insight into the mechanical function of the coronoid process. The salient findings include (1) isolated injury to more than 50 percent of the coronoid process can produce axial elbow instability; (2) the coronoid process seems to function as an anterior buttress to prevent posterior elbow subluxation, and this role may be particularly important between 60 and 105 degrees of flexion; and (3) fractures involving more than 50 percent of the coronoid process behave differently biomechanically than those involving 50 percent or less of the coronoid process.

The stability of the elbows with a type-I or type-II fracture was demonstrated to be similar to that of the controls, and this is in keeping with clinical observations of these injuries^21,22,26. Subluxation of the elbows with a type-III fracture under minimal loading supports the clinical finding that untreated type-III fractures of the coronoid process are associated with a poor functional outcome^26,27. We realize that type-III injuries are rare and often are associated with additional elbow injuries. However, case reports of isolated fractures have demonstrated that these injuries are unstable and that, when they are reduced and stabilized, the outcome can be satisfactory^{10,11,13,17,24,25,28,30}.

The data from our study also suggest that the coronoid process has a role as a buttress to posterior ulnar displacement, and this role may be dependent on the amount of elbow flexion. This suggestion is consistent with the findings of prior elbow studies. Morrey et al. reported that the transmission of forces through the radial head are greatest between 0 and 30 degrees of flexion²⁰. It appears that, as the elbow continues to flex, the transmission shifts to the coronoid process. We found that, with the loaded type-III fractures, there was consistent posterior subluxation when the elbow was flexed between 60 and 105 degrees. This appears to indirectly support the idea that the coronoid process is an important stabilizer against posterior axial displacement at positions of elbow flexion of 60 degrees and more.

Regarding our original hypothesis, it seems that isolated injuries of the coronoid process can cause instability. We recognize that these fractures are rare. In this study, we did not attempt to model common clinical injuries but, rather, we investigated only the mechanical role of the coronoid process. Unfortunately, this study does not fully explain why type-III coronoid fractures behave differently clinically than type-I and type-II injuries.

In summary, this investigation provides some biomechanical support for the current treatment of coronoid process injuries. The results demonstrate that, in response to an axial load, fractures involving more than 50 percent of the coronoid process behave differently biomechanically than injuries involving less than 50 percent of the coronoid process. Also, it appears that the coronoid process may be particularly important as a stabilizing structure during elbow flexion between 60 and 105 degrees. The data support the current classification and treatment of coronoid process fractures - that is, early motion may be allowed when a patient has a type-I or type-II isolated injury of the coronoid process as there appears to be little risk of posterior subluxation. However, even isolated type-III injuries appear to be at risk for posterior subluxation under minimal physiological loads, particularly with elbow flexion between 60 and 105 degrees. This finding supports the trend toward open reduction and internal fixation of these injuries.

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