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Stability of the elbow: Osseous constraints
Stability of the elbow: Osseous constraints Bernard F. Morrey, MD,a and Kai-Nan An, PhD,b Rochester, MN
The elbow is considered a complex joint, and thus the elements contributing to stability of the elbow are no less intricate. The soft-tissue and articular surfaces that provide stability to the elbow share the capacity as a function of joint position and loading configuration. The issue of soft-tissue constraints is dealt with elsewhere in this volume, and the focus of this discussion is that of the articular contributions. In this context, the radial head is defined as a secondary stabilizer to resist valgus force. The coronoid is clearly the most important articular stabilizer of this joint. (J Shoulder Elbow Surg 2005;14:174S-178S.)
O ne of the earliest means of studying the articular
contribution to elbow stabilization consists of measuring the displacement at the joint based on serial alterations of the stabilizing elements, typically by use of force displacement experimental methodologies (Figure 1). This approach is sensitive to the test fixtures, which can introduce bias into the assessment by limiting the degrees of freedom or paths of the displacement. From a clinical standpoint, the stability of the elbow is viewed in the context of function, which introduces a dynamic element into the equation. Thus, those methods that assess elbow stability in simulated dynamic loading are generally considered more clinically relevant and reliable. Such techniques allow assessment during continuous elbow flexion, which is critically important for accurately understanding the stabilizing restraints. Finally, the sensitivity and accuracy of the measurement of both angular and translational displacement are particularly important. Devices that use NASA technology of electromagnetic telemetry satisfy the criteria noted above to provide accurate and simultaneous measurement of rotatory and translatory displacement (Figure 2).
From the aDepartment of Orthopedic Surgery and bOrthopedic Biomechanics Laboratory, Mayo Clinic. Reprint requests: Bernard F. Morrey, MD, Mayo Clinic, 200 First St, SW, Rochester MN 55905 (E-mail: morrey.bernard@mayo. edu). Copyright © 2005 by Journal of Shoulder and Elbow Surgery Board of Trustees. 1058-2746/2005/$30.00 doi:10.1016/j.jse.2004.09.031
174S
ARTICULAR CONTRIBUTIONS TO ELBOW STABILITY To fully understand the relative contributions of the articulation in providing stability, it is first important to understand the force distribution across the joint under various loading conditions. This was studied several years ago, with findings that showed that approximately 60% of an axial load imparted to the wrist is transmitted across the radiohumeral joint and 40% across the ulnohumeral joint (Figure 3).3 Subsequent investigations have suggested that approximately an equal amount of force is transmitted across the trochlea and the capitellum when the elbow is in flexion (Figure 4). When the elbow is in extension, 40% to 50% more force is transmitted across the trochlea than through the radiohumeral joint (Figure 5). This issue of articular stabilization is best treated by sequentially assessing the olecranon component of the ulnohumeral joint, the radiohumeral articulation, and the coronoid component of the joint. Ulnohumeral articulation: Olecranon
Clinically, it is recognized that portions of the olecranon, when fractured, can be removed without substantively altering function. Surprisingly, it has even been demonstrated, clinically, that extension strength is maintained in the presence of an absent proximal ulna.2 Relative stability of the ulnohumeral joint has been assessed in our laboratory by serial resection of the proximal olecranon in segments involving 25%, 50%, and 75% of the ulnar articulation and then the entire ulnar articulation.1 The loading configuration was one of a compressive longitudinal load along the long axis of the humerus coupled with an internal and external torsion as well. The elbow was tested in both full extension and 90° of flexion. By use of a force displacement methodology, it was demonstrated that the proximal ulna contributes to the stability of the joint in a linear fashion as a function of the amount present. Thus, 25% resection of the proximal ulna decreases the static stability of this articulation by about 25% (Figure 6) in both flexion and extension. On the basis of this assessment, it is believed that 50% of this joint can be removed without adversely affecting joint stability. This fact is used to treat some severe comminuted fractures in the elderly.
J Shoulder Elbow Surg Volume 14, Number 1S
Morrey and An
175S
Figure 1 A, Photograph of elbow force displacement testing setup. B, The force displacement curve demonstrates alteration in the amount of force required for a given displacement. This allows the attribution of a percentage of overall contribution to stability of the part altered between experiments. Although this is a relatively simple method, the disadvantage is that the procedure generally restricts motion and does not replicate physiologic circumstances.
Figure 2 The magnetic tracking device allows 3-dimensional rotatory and 3-dimensional translatory motion to be monitored by a fixed source referable to a moving sensor. By simulating the motor function of the muscles across the joint, accurate replications of kinematic and force distribution may be calculated.
Radiohumeral joint
The contribution of the radial head to valgus stability has been the clinical circumstance most frequently studied in the experimental setting. The issue is typically investigated and discussed in the context of various types of radial head replacements and their material properties.4 –10 The method of studying the ligamentous and articular contributions to stability is most commonly assessed by imparting a specific and controlled displacement to the elbow in a set position. The relative contribution of each stabilizing element is then measured by correlating the resistive force generated in the load cell after sequentially altering or eliminating the various contributions to stability. An alternative
Figure 3 It has been estimated that approximately 60% of the axial load is transmitted across the radiohumeral joint when the elbow is in full extension.
technique is to observe the alteration in displacement after introducing a specific load to the system. In our experience, the former has proved to be the more reliable method when the relative stabilizing contribution is a function of the sequence of alteration of the constraints.
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Figure 4 In flexion, the amount of force transmitted across the capitellum and trochlea is approximately equal.
J Shoulder Elbow Surg January/February 2005
Figure 6 With the elbow flexed and extended, the stability of the elbow is proportionately altered as a function of the amount of olecranon resected; hence, approximately 50% of the stabilizing force of the ulna to resist axial displacement and rotation is lost by removing the proximal 50% of the articulation.
Figure 5 In extension, a greater amount of force is recorded across the ulnohumeral joint than across the radiohumeral joint.
In the past, the use of this methodology to study the ligaments and articular elements has tended to overestimate the role of the radial head in resisting valgus load.4,7 A more accurate means of assessing the contribution of the radial head to valgus displacement involves the use of an electromagnetic tracking device as described above. By use of this methodology, the role of the radial head is more accurately demonstrated in various positions and for specific loading configurations. By placing the elbow in the so-called gravity valgus position, accurate measurement of angular and rotatory displacement is possible through a simulated arc of active flexion (Figure 2). By first removing the radial head, very little alteration of valgus displacement occurs when the medial collateral complex is intact (Figure 7). However, once the medial collateral ligament is removed, the elbow is shown to sublux in the absence of a radial head. Reversing the experiment by first removing the medial collateral ligament does demonstrate some valgus instability. However,
subsequently removing the radial head results in subluxation. When the collateral ligament and the soft tissue of the forearm are intact, the radial head may be removed with some degree of impunity. If, however, either the distal radioulnar joint has been violated or a medial or lateral collateral ligament has been disrupted, then radial head integrity is critical to elbow function. This experiment clearly defines the radial head as a secondary stabilizer of the elbow in resisting valgus stress, thus defining the rationale and indications for radial head replacement. Coronoid
Clinical recognition of the coronoid in providing elbow stability is now well realized. The relative contribution of the coronoid and radial head to posterior displacement of the forearm has recently been assessed. From a functional perspective, both the flexor mus-
J Shoulder Elbow Surg Volume 14, Number 1S
Morrey and An
177S
Figure 7 A, Removing the radial head (RH) and placing the elbow in valgus when the medial collateral ligament (MCL) is intact results in relatively little displacement of the forearm. When the medial collateral ligament is then removed, marked instability is demonstrated. B, On the other hand, when the sequence is altered and the medial collateral ligament is released, some valgus instability is noted. After this, removal of the radial head results in subluxation of the elbow. This defines the radial head as an important secondary stabilizer of the elbow to resist valgus stress.
culature and the extensor musculature have a posteriorly directed component. Thus, whether the elbow is being flexed or extended, the forearm has a tendency to displace posteriorly (Figure 8). This explains the propensity for the forearm to sublux posteriorly under virtually all circumstances, especially when the anterior articulation is deficient. This issue was studied in our laboratory by releasing all soft-tissue elements and measuring displacement with the electromagnetic sensor. Serial amounts of the coronoid measuring 25%, 50%, 75%, and 100% were then removed, and simulated active motion from flexion to extension was introduced. This experiment documented that the greatest stability is in flexion and that at least 50% of the coronoid is necessary to provide a functionally stable ulnohumeral joint near extension (Figure 9, A). The experiment was repeated with the radial head removed. In this setting, the elbow was found to be grossly unstable even when flexed past 100°. With 50% of the coronoid present, however, the elbow does demonstrate some degree of stability until the elbow nears extension (Figure 9, B). On the basis of this experiment, it may be concluded that the major stabilizer of the ulnohumeral joint is the coronoid. The tendency for resisting posterior displacement is markedly enhanced by the presence of an intact radial head. Our experiments indicate that a minimum of 50% of the coronoid is necessary to have any chance of functional elbow stability. From a clinical perspective, we have ob-
Figure 8 With the elbow flexing and extending, the major motors that flex and extend the joint (triceps [TR], brachialis [BR], and biceps [BC]) all have a component of the resultant vector of the active contraction directed posteriorly. This helps explain why the elbow is subluxed posteriorly when there is articular deficiency anteriorly, usually of the coronoid.
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Morrey and An
J Shoulder Elbow Surg January/February 2005
Figure 9 A, After 75% of the coronoid is removed, but with an intact radial head, the elbow is stable from flexion to about 30° of full extension. With 50% of the coronoid removed, posterior displacement is resisted when the radial head is intact, even near full extension. B, When the radial head is removed, dramatically more posterior instability is exhibited under all testing conditions.
served what we might term a “minimal required articulation” to be 50% of the coronoid. This may be assessed clinically by observing that when a line is drawn from the tip of the olecranon parallel to the long axis of the ulna, it bisects approximately 50% of the coronoid. SUMMARY On the basis of our interpretation of these experimental results, as well as our clinical experience, the concept of the primacy of the coronoid as the major articular contribution to the elbow has been developed. It is concluded that the olecranon is the least important component and 50% may be absent without affecting elbow function in any clinically relevant manner. The second least important, as an isolated contribution, is the radial head, as its absence in the face of other elements being present is functionally quite minimal. On the other hand, its role as a secondary stabilizer is critically important. The most important articular component of elbow stability is the coronoid. At least 50% of the coronoid must be present to provide stability to the elbow joint regardless of the integrity of the other articular elements.
REFERENCES
1. An KN, Morrey BF, Chao EY. The effect of partial removal of proximal ulna on elbow constraint. Clin Orthop 1986;209: 270-9. 2. Gartsman GM, Sculco TP, Otis JC. Operative treatment of olecranon fractures. Excision or open reduction with internal fixation. J Bone Joint Surg Am 1981;63:718-21. 3. Halls AA, Travill A. Transmission of pressures across the elbow joint. Anat Rec 1964;150:243-7. 4. Hotchkiss RN, Weiland AJ. Valgus stability of the elbow. J Orthop Res 1987;5:372-7. 5. Jensen SL, Olsen BS, Sojbjerg JO. Elbow joint kinematics after excision of the radial head. J Shoulder Elbow Surg 1999;8:23841. 6. King GJ, Zarzour ZD, Rath DA, et al. Metallic radial head arthroplasty improves valgus stability of the elbow. Clin Orthop 1999;368:114-25. 7. Morrey BF, Tanaka S, An KN. Valgus stability of the elbow. A definition of primary and secondary constraints. Clin Orthop 1991;265:187-95. 8. Pomianowski S, Morrey BF, Neale PG, et al. Contribution of monoblock and bipolar radial head prostheses to valgus stability of the elbow. J Bone Joint Surg Am 2001;83:1829-34. 9. Pribyl CR, Kester MA, Cook SD, Edmunds JO, Brunet ME. The effect of the radial head and prosthetic radial head replacement on resisting valgus stress at the elbow. Orthopedics 1986;9: 723-6. 10. Sojbjerg JO, Ovesen J, Gundorf CE. The stability of the elbow following excision of the radial head and transection of the annular ligament. Arch Orthop Trauma Surg 1987;106: 248-50.
The elbow is considered a complex joint, and thus the elements contributing to stability of the elbow are no less intricate. The soft-tissue and articular surfaces that provide stability to the elbow share the capacity as a function of joint position and loading configuration. The issue of soft-tissue constraints is dealt with elsewhere in this volume, and the focus of this discussion is that of the articular contributions. In this context, the radial head is defined as a secondary stabilizer to resist valgus force. The coronoid is clearly the most important articular stabilizer of this joint. (J Shoulder Elbow Surg 2005;14:174S-178S.)
O ne of the earliest means of studying the articular
contribution to elbow stabilization consists of measuring the displacement at the joint based on serial alterations of the stabilizing elements, typically by use of force displacement experimental methodologies (Figure 1). This approach is sensitive to the test fixtures, which can introduce bias into the assessment by limiting the degrees of freedom or paths of the displacement. From a clinical standpoint, the stability of the elbow is viewed in the context of function, which introduces a dynamic element into the equation. Thus, those methods that assess elbow stability in simulated dynamic loading are generally considered more clinically relevant and reliable. Such techniques allow assessment during continuous elbow flexion, which is critically important for accurately understanding the stabilizing restraints. Finally, the sensitivity and accuracy of the measurement of both angular and translational displacement are particularly important. Devices that use NASA technology of electromagnetic telemetry satisfy the criteria noted above to provide accurate and simultaneous measurement of rotatory and translatory displacement (Figure 2).
From the aDepartment of Orthopedic Surgery and bOrthopedic Biomechanics Laboratory, Mayo Clinic. Reprint requests: Bernard F. Morrey, MD, Mayo Clinic, 200 First St, SW, Rochester MN 55905 (E-mail: morrey.bernard@mayo. edu). Copyright © 2005 by Journal of Shoulder and Elbow Surgery Board of Trustees. 1058-2746/2005/$30.00 doi:10.1016/j.jse.2004.09.031
174S
ARTICULAR CONTRIBUTIONS TO ELBOW STABILITY To fully understand the relative contributions of the articulation in providing stability, it is first important to understand the force distribution across the joint under various loading conditions. This was studied several years ago, with findings that showed that approximately 60% of an axial load imparted to the wrist is transmitted across the radiohumeral joint and 40% across the ulnohumeral joint (Figure 3).3 Subsequent investigations have suggested that approximately an equal amount of force is transmitted across the trochlea and the capitellum when the elbow is in flexion (Figure 4). When the elbow is in extension, 40% to 50% more force is transmitted across the trochlea than through the radiohumeral joint (Figure 5). This issue of articular stabilization is best treated by sequentially assessing the olecranon component of the ulnohumeral joint, the radiohumeral articulation, and the coronoid component of the joint. Ulnohumeral articulation: Olecranon
Clinically, it is recognized that portions of the olecranon, when fractured, can be removed without substantively altering function. Surprisingly, it has even been demonstrated, clinically, that extension strength is maintained in the presence of an absent proximal ulna.2 Relative stability of the ulnohumeral joint has been assessed in our laboratory by serial resection of the proximal olecranon in segments involving 25%, 50%, and 75% of the ulnar articulation and then the entire ulnar articulation.1 The loading configuration was one of a compressive longitudinal load along the long axis of the humerus coupled with an internal and external torsion as well. The elbow was tested in both full extension and 90° of flexion. By use of a force displacement methodology, it was demonstrated that the proximal ulna contributes to the stability of the joint in a linear fashion as a function of the amount present. Thus, 25% resection of the proximal ulna decreases the static stability of this articulation by about 25% (Figure 6) in both flexion and extension. On the basis of this assessment, it is believed that 50% of this joint can be removed without adversely affecting joint stability. This fact is used to treat some severe comminuted fractures in the elderly.
J Shoulder Elbow Surg Volume 14, Number 1S
Morrey and An
175S
Figure 1 A, Photograph of elbow force displacement testing setup. B, The force displacement curve demonstrates alteration in the amount of force required for a given displacement. This allows the attribution of a percentage of overall contribution to stability of the part altered between experiments. Although this is a relatively simple method, the disadvantage is that the procedure generally restricts motion and does not replicate physiologic circumstances.
Figure 2 The magnetic tracking device allows 3-dimensional rotatory and 3-dimensional translatory motion to be monitored by a fixed source referable to a moving sensor. By simulating the motor function of the muscles across the joint, accurate replications of kinematic and force distribution may be calculated.
Radiohumeral joint
The contribution of the radial head to valgus stability has been the clinical circumstance most frequently studied in the experimental setting. The issue is typically investigated and discussed in the context of various types of radial head replacements and their material properties.4 –10 The method of studying the ligamentous and articular contributions to stability is most commonly assessed by imparting a specific and controlled displacement to the elbow in a set position. The relative contribution of each stabilizing element is then measured by correlating the resistive force generated in the load cell after sequentially altering or eliminating the various contributions to stability. An alternative
Figure 3 It has been estimated that approximately 60% of the axial load is transmitted across the radiohumeral joint when the elbow is in full extension.
technique is to observe the alteration in displacement after introducing a specific load to the system. In our experience, the former has proved to be the more reliable method when the relative stabilizing contribution is a function of the sequence of alteration of the constraints.
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Morrey and An
Figure 4 In flexion, the amount of force transmitted across the capitellum and trochlea is approximately equal.
J Shoulder Elbow Surg January/February 2005
Figure 6 With the elbow flexed and extended, the stability of the elbow is proportionately altered as a function of the amount of olecranon resected; hence, approximately 50% of the stabilizing force of the ulna to resist axial displacement and rotation is lost by removing the proximal 50% of the articulation.
Figure 5 In extension, a greater amount of force is recorded across the ulnohumeral joint than across the radiohumeral joint.
In the past, the use of this methodology to study the ligaments and articular elements has tended to overestimate the role of the radial head in resisting valgus load.4,7 A more accurate means of assessing the contribution of the radial head to valgus displacement involves the use of an electromagnetic tracking device as described above. By use of this methodology, the role of the radial head is more accurately demonstrated in various positions and for specific loading configurations. By placing the elbow in the so-called gravity valgus position, accurate measurement of angular and rotatory displacement is possible through a simulated arc of active flexion (Figure 2). By first removing the radial head, very little alteration of valgus displacement occurs when the medial collateral complex is intact (Figure 7). However, once the medial collateral ligament is removed, the elbow is shown to sublux in the absence of a radial head. Reversing the experiment by first removing the medial collateral ligament does demonstrate some valgus instability. However,
subsequently removing the radial head results in subluxation. When the collateral ligament and the soft tissue of the forearm are intact, the radial head may be removed with some degree of impunity. If, however, either the distal radioulnar joint has been violated or a medial or lateral collateral ligament has been disrupted, then radial head integrity is critical to elbow function. This experiment clearly defines the radial head as a secondary stabilizer of the elbow in resisting valgus stress, thus defining the rationale and indications for radial head replacement. Coronoid
Clinical recognition of the coronoid in providing elbow stability is now well realized. The relative contribution of the coronoid and radial head to posterior displacement of the forearm has recently been assessed. From a functional perspective, both the flexor mus-
J Shoulder Elbow Surg Volume 14, Number 1S
Morrey and An
177S
Figure 7 A, Removing the radial head (RH) and placing the elbow in valgus when the medial collateral ligament (MCL) is intact results in relatively little displacement of the forearm. When the medial collateral ligament is then removed, marked instability is demonstrated. B, On the other hand, when the sequence is altered and the medial collateral ligament is released, some valgus instability is noted. After this, removal of the radial head results in subluxation of the elbow. This defines the radial head as an important secondary stabilizer of the elbow to resist valgus stress.
culature and the extensor musculature have a posteriorly directed component. Thus, whether the elbow is being flexed or extended, the forearm has a tendency to displace posteriorly (Figure 8). This explains the propensity for the forearm to sublux posteriorly under virtually all circumstances, especially when the anterior articulation is deficient. This issue was studied in our laboratory by releasing all soft-tissue elements and measuring displacement with the electromagnetic sensor. Serial amounts of the coronoid measuring 25%, 50%, 75%, and 100% were then removed, and simulated active motion from flexion to extension was introduced. This experiment documented that the greatest stability is in flexion and that at least 50% of the coronoid is necessary to provide a functionally stable ulnohumeral joint near extension (Figure 9, A). The experiment was repeated with the radial head removed. In this setting, the elbow was found to be grossly unstable even when flexed past 100°. With 50% of the coronoid present, however, the elbow does demonstrate some degree of stability until the elbow nears extension (Figure 9, B). On the basis of this experiment, it may be concluded that the major stabilizer of the ulnohumeral joint is the coronoid. The tendency for resisting posterior displacement is markedly enhanced by the presence of an intact radial head. Our experiments indicate that a minimum of 50% of the coronoid is necessary to have any chance of functional elbow stability. From a clinical perspective, we have ob-
Figure 8 With the elbow flexing and extending, the major motors that flex and extend the joint (triceps [TR], brachialis [BR], and biceps [BC]) all have a component of the resultant vector of the active contraction directed posteriorly. This helps explain why the elbow is subluxed posteriorly when there is articular deficiency anteriorly, usually of the coronoid.
178S
Morrey and An
J Shoulder Elbow Surg January/February 2005
Figure 9 A, After 75% of the coronoid is removed, but with an intact radial head, the elbow is stable from flexion to about 30° of full extension. With 50% of the coronoid removed, posterior displacement is resisted when the radial head is intact, even near full extension. B, When the radial head is removed, dramatically more posterior instability is exhibited under all testing conditions.
served what we might term a “minimal required articulation” to be 50% of the coronoid. This may be assessed clinically by observing that when a line is drawn from the tip of the olecranon parallel to the long axis of the ulna, it bisects approximately 50% of the coronoid. SUMMARY On the basis of our interpretation of these experimental results, as well as our clinical experience, the concept of the primacy of the coronoid as the major articular contribution to the elbow has been developed. It is concluded that the olecranon is the least important component and 50% may be absent without affecting elbow function in any clinically relevant manner. The second least important, as an isolated contribution, is the radial head, as its absence in the face of other elements being present is functionally quite minimal. On the other hand, its role as a secondary stabilizer is critically important. The most important articular component of elbow stability is the coronoid. At least 50% of the coronoid must be present to provide stability to the elbow joint regardless of the integrity of the other articular elements.
REFERENCES
1. An KN, Morrey BF, Chao EY. The effect of partial removal of proximal ulna on elbow constraint. Clin Orthop 1986;209: 270-9. 2. Gartsman GM, Sculco TP, Otis JC. Operative treatment of olecranon fractures. Excision or open reduction with internal fixation. J Bone Joint Surg Am 1981;63:718-21. 3. Halls AA, Travill A. Transmission of pressures across the elbow joint. Anat Rec 1964;150:243-7. 4. Hotchkiss RN, Weiland AJ. Valgus stability of the elbow. J Orthop Res 1987;5:372-7. 5. Jensen SL, Olsen BS, Sojbjerg JO. Elbow joint kinematics after excision of the radial head. J Shoulder Elbow Surg 1999;8:23841. 6. King GJ, Zarzour ZD, Rath DA, et al. Metallic radial head arthroplasty improves valgus stability of the elbow. Clin Orthop 1999;368:114-25. 7. Morrey BF, Tanaka S, An KN. Valgus stability of the elbow. A definition of primary and secondary constraints. Clin Orthop 1991;265:187-95. 8. Pomianowski S, Morrey BF, Neale PG, et al. Contribution of monoblock and bipolar radial head prostheses to valgus stability of the elbow. J Bone Joint Surg Am 2001;83:1829-34. 9. Pribyl CR, Kester MA, Cook SD, Edmunds JO, Brunet ME. The effect of the radial head and prosthetic radial head replacement on resisting valgus stress at the elbow. Orthopedics 1986;9: 723-6. 10. Sojbjerg JO, Ovesen J, Gundorf CE. The stability of the elbow following excision of the radial head and transection of the annular ligament. Arch Orthop Trauma Surg 1987;106: 248-50.