The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 86, Issue 10

Scientific Articles | October 01, 2004

Effect of Distal Humeral Varus Deformity on Strain in the Lateral Ulnar Collateral Ligament and Ulnohumeral Joint Stability

Murray J. Beuerlein, MSc¹; Jeffrey T. Reid, MD¹; Emil H. Schemitsch, MD, FRCS(C)¹; Michael D. McKee, MD, FRCS(C)¹

¹ Upper Extremity Reconstructive Service (M.J.B., J.T.R., and M.D. McK.) and Division of Orthopaedics (E.H.S.), St. Michael's Hospital and the University of Toronto, 55 Queen Street East, Suite 800, Toronto, ON M5C 1R6, Canada. E-mail address for M.D. McKee: mckee@the-wire.com

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The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at St. Michael's Hospital and the University of Toronto, Toronto, Ontario, Canada

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2004 Oct 01;86(10):2235-2242

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Abstract

Background: Recent reports have implicated cubitus varus deformity as a risk factor for the development of late posterolateral rotatory instability of the elbow. The purpose of this study was to determine the biomechanical relationship between cubitus varus and strain in the lateral ulnar collateral ligament and subluxation of the ulnohumeral joint as it relates to posterolateral rotatory instability.

Methods: Eleven fresh-frozen cadaveric elbow joints were mounted in an apparatus that allowed active elbow motion. Supracondylar osteotomies were performed to create cubitus varus deformities of 0° to 30° in 5° increments. Each elbow was loaded with a supination moment of 1.2 Nm and a resistive triceps extension force. Strain in the lateral ulnar collateral ligament and widening of the ulnohumeral joint were measured at each position of varus, and the resulting strain and joint-widening values were averaged.

Results: Strain in the lateral ulnar collateral ligament was found to increase as the cubitus varus deformity increased. Likewise, the ulnohumeral joint space was seen to progressively widen with increasing cubitus varus. Strain was significantly increased at 30° of varus deformity (p = 0.03), and widening of the ulnohumeral joint space was significantly increased at 25° of varus deformity (p = 0.004). When differences in the size of the cadaveric specimens were accounted for in an analysis of covariance, ligament strain was significantly increased at 25° of varus (p = 0.005) and widening of the ulnohumeral joint space, at 20° (p = 0.01).

Conclusions: Cubitus varus deformity of the elbow increases strain in the lateral ulnar collateral ligament, with a corresponding increase in ulnohumeral joint-opening consistent with the posterolateral rotatory instability of the elbow seen clinically.

Clinical Relevance: The results of this study are consistent with the clinical findings of posterolateral rotatory instability of the elbow following long-standing cubitus varus and support the clinical concept that increased strain in the lateral ulnar collateral ligament with this deformity may eventually lead to overt posterolateral rotatory instability of the elbow. We believe that these findings have implications for the etiology, prevention, and treatment of posterolateral rotatory instability of the elbow following cubitus varus deformity.

Figures in this Article

Posterolateral rotatory instability of the elbow was characterized by O'Driscoll et al. in 1991¹. It is distinguished by rotatory subluxation of the ulnohumeral joint and associated subluxation of the radiohumeral joint^1-5, and it is diagnosed clinically with use of the lateral pivot shift test^1,5-8. The pathology underlying posterolateral rotatory instability of the elbow is deficiency of the lateral stabilizers of the elbow and, in particular, the lateral ulnar collateral ligament. The lateral ulnar collateral ligament has been described as the primary constraint to posterolateral rotatory instability of the elbow, and sectioning of this ligament has been shown to result in a positive lateral pivot shift test². Furthermore, attenuation of the lateral ulnar collateral ligament has been consistently described during operative repairs to address posterolateral rotatory instability of the elbow, and repair or reconstruction of the lateral ulnar collateral ligament has led to resolution of the instability^4,5.

Recently, posterolateral rotatory instability of the elbow has been described in patients after long-standing cubitus varus deformity following malunion of a supracondylar distal humeral fracture^4,9-13. O'Driscoll et al. reported on a series of twenty-five elbows with cubitus varus deformity and subsequent posterolateral rotatory instability of the elbow seen at multiple centers in multiple countries⁴. In that series, the varus deformity averaged 15°, and the symptoms of instability became clinically evident decades after the development of the varus deformity. The mechanism for this late instability is unknown. It has been postulated to involve changes in elbow biomechanics such that the lateral ulnar collateral ligament is repeatedly subjected to increased strain, which leads to attenuation and stretching, a decreased ability to withstand physiologic loading, and eventually posterolateral rotatory instability of the elbow following secondary trauma⁴.

The purpose of this study was to determine the biomechanical relationship between cubitus varus and strain in the lateral ulnar collateral ligament. In addition, we measured the effect of cubitus varus deformity on the development of ulnohumeral joint-widening as an indicator of clinical posterolateral rotatory instability of the elbow.

Materials and Methods

Materials and Methods | Results | Discussion | References

Eleven fresh-frozen cadaveric upper extremities were used. The specimens were obtained from six male and five female donors who had been more than sixty-five years of age at the time of death. Five right and six left extremities were used. All specimens were determined to be free of pathological changes in the elbow with clinical examination and gross inspection. Prior to dissection, each specimen was thawed overnight at room temperature. The specimens were stored, used, and disposed of in accordance with the University of Toronto guidelines for the use of human specimens and the Anatomy Act of Ontario.

Each specimen was dissected free of all soft tissues except the elbow joint capsule and surrounding ligamentous structures, the interosseous ligament, and the soft tissues around the distal radioulnar joint. The lateral ulnar collateral ligament was identified in each specimen. A small arthrotomy was made well posterior to the lateral ulnar collateral ligament to observe the ulnohumeral joint. Cohen and Hastings¹⁴ reported that the thin joint capsule in this area has little stabilizing effect on the elbow. Overall, any minor structural deficit induced by this procedure was consistent between specimens and was considered unlikely to influence stability. The humerus was sectioned 25 cm proximal to the elbow joint, and the wrist was disarticulated at the radiocarpal joint. The distal radioulnar joint was left intact.

Each specimen was mounted in an apparatus that allowed active triceps extension to counteract a coupled flexion and supination force applied to the ulna (Figs. 1-A and 1-B). A supracondylar 30° medial closing wedge osteotomy was performed on all specimens at the distal humeral metaphysis, 15 mm proximal to the transepicondylar axis. The osteotomy did not involve the joint capsule or adjacent ligamentous structures in any of the specimens. Bone wedges of 5° to 25° in 5° increments were then placed in the osteotomy site to allow for a final cubitus varus deformity of 0° to 30°. The osteotomy sites were secured with small-fragment screws and plates on the medial and lateral columns. All osteotomy sites were stable to varus, valgus, flexion, extension, and torsional loading once they had been secured. Stability was defined as the absence of any visible motion at the osteotomy site with loading by the apparatus.

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Figs. 1-A and 1-B: Experimental apparatus with the cadaveric upper extremity mounted during the testing protocol. Fig. 1-A A lateral view of the apparatus showing the biceps flexion force (B) and triceps extension force (T) acting across the elbow joint.

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Fig. 1-B: The apparatus shown from above. Note the position of the radius (R) and ulna (U) and the supination moment of 1.2 Nm (S).

A differential variable reluctance transducer (DVRT-3; MicroStrain, Burlington, Vermont) was sutured to the lateral ulnar collateral ligament in a standard location in its midsubstance. The transducer was calibrated on a micrometer stage, and linear regression analysis was used to determine the calibration coefficients. Error analysis of the transducer revealed ±0.018-mm maximal error over a 3-mm working range, or ±0.6% error, which is highly linear over this range (r² = 1.0). This resulted in a maximal strain error of 0.00133 (0.133%). The zero-strain length of the ligament was then determined by applying incremental pronation and supination moments with the elbow flexed to 35°. This was repeated at each position of cubitus varus throughout the experiment, or seven times for each elbow. The seven resulting estimates of the zero-strain length of the ligament were then averaged to provide a single estimate of zero-strain length for each ligament in the study. This technique provided a curve representing the average pronation-supination moment versus lateral ulnar collateral ligament length. The inflection point of the curve was taken as zero-strain length. This technique was previously described and validated¹⁵.

Cubitus varus osteotomies were performed sequentially but in random order of degrees of varus from specimen to specimen. At each given position of varus, following determination of the zero-strain length, the elbow was loaded with a supination moment of 1.2 Nm and a resistive triceps extension force. The moment was generated by hanging a 1.2-kg mass at a displacement of 0.12 m from the center of rotation of the ulna while the elbow was flexed to 35°. The pure supination moment about the axis of the ulna was calculated by resolving the force vectors acting in that plane (cos 35° * 1.2 kg * 9.81 m/s² * 0.12 m = 1.2 Nm). The moment was less than the physiologic moments on the elbow, which have been shown to be 4 to 8 Nm of supination for falls onto an outstretched hand from a height of 3 to 6 cm¹⁶. In other studies, flexion moments about the elbow have been reported to range from 6 to 11 Nm for routine activities of daily living¹⁷. We chose a lesser load since our model was devoid of all dynamic stabilizers that attenuate the load in vivo. As a result, in our model the entire load was transmitted directly to the static stabilizers such as the ligaments. For this reason, we expected the ligaments to be strained at a simulated physiologic load that is lower than the load that would strain a native elbow with fully functioning soft tissues.

The triceps force was adjusted such that the elbow rested at 35° of flexion. This angle of flexion was selected because it produces the maximal rotatory displacement of the elbow clinically¹. The strain in the lateral ulnar collateral ligament was then measured with the previously described zero-strain length as a reference. The lateral aspect of the ulnohumeral joint was observed to widen or open to a variable degree with loading, and a handheld digital micrometer (Mitutoyo Absolute Digimatic Caliper, model CD-6" CS; Mitutoyo, Kawasaki, Kanagawa, Japan) was used to measure this ulnohumeral distance at its widest point (Fig. 2). The manufacturer's specifications for this device are a resolution of 0.01 mm, accuracy of ±0.02 mm, and repeatability of 0.01 mm. This opening is indicative of posterolateral rotatory instability of the elbow, as the ulna rotates away from the distal part of the humerus in the posterolateral direction. Three iterations were performed at each position of varus, and the resulting strain and joint-widening values were averaged. This protocol did not test the ligament to failure.

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Fig. 2: Measurement of ulnohumeral joint subluxation. A handheld micrometer was used to measure the ulnohumeral subluxation at its widest point. Three iterations were performed at each degree of varus deformity, and the values were averaged. U = ulna, DH = distal part of the humerus, and RH = radial head.

To test the reproducibility of the handheld digital micrometer for measuring ulnohumeral joint-widening, the magnitude of the gap was measured thirty times at each position of varus deformity in a single elbow. This was performed by a single observer who was blinded to the result of each measurement. The mean and standard deviation was calculated for each position (Table I). The standard deviation of the measurement was 0.05 to 0.06 mm for all positions of cubitus varus deformity.

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TABLE I Repeatability of the Measurements of Ulnohumeral Joint-Widening

	Cubitus Varus
	0°	5°	10°	15°	20°	25°	30°
Average	0.71	0.94	1.17	1.33	1.65	1.98	2.64
Standard deviation	0.05	0.05	0.05	0.05	0.05	0.06	0.06

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TABLE I Repeatability of the Measurements of Ulnohumeral Joint-Widening

	Cubitus Varus
	0°	5°	10°	15°	20°	25°	30°
Average	0.71	0.94	1.17	1.33	1.65	1.98	2.64
Standard deviation	0.05	0.05	0.05	0.05	0.05	0.06	0.06

After the 30° medial closing wedge osteotomy was performed, the ulnohumeral angle was measured to determine the relative varus created at the elbow joint and to calculate the initial carrying angle of the specimens. This was performed by a single observer using a handheld goniometer placed parallel to the long axes of the humerus and ulna.

The size of each specimen was quantified by measuring the width of the specimen at the distal humeral epicondyles. The transepicondylar width was measured with use of the digital handheld micrometer.

Statistical analysis of strain and joint-widening relative to cubitus varus deformity was performed with repeated-measures analysis of variance and with analysis of covariance with transepicondylar width as the covariant. Transepicondylar width was incorporated in the analysis to correct for any size differences among the specimens.

Correlation between varus deformity and ulnohumeral joint-widening or strain was calculated with the Pearson correlation coefficient. Differences in specimen size between genders were evaluated with analysis of variance. Comparison of the carrying angle between genders was carried out with use of the Student t test.

Results

Materials and Methods | Results | Discussion | References

The lateral ulnar collateral ligament was visualized in all specimens, and our protocol did not lead to failure of any of the ligaments tested.

The average carrying angle (and standard deviation) was 5.1° ± 1.4° with a range of 3° to 8°. The male specimens had an average carrying angle of 4.7° compared with 5.6° for the female specimens (p = 0.31).

The transepicondylar widths of the eleven cadaveric specimens ranged from 56 to 74 mm. The male specimens had a consistently larger transepicondylar width than did the female specimens, and analysis of variance revealed this difference to be significant (p < 0.001).

Strain in the lateral ulnar collateral ligament was found to increase, at a supination moment of 1.2 Nm, as the cubitus varus deformity increased (Fig. 3, r = 0.429, p < 0.01). As compared with the value in the undisturbed native elbow, there was a significant increase in the strain in the lateral ulnar collateral ligament when a 30° varus cubitus deformity was achieved (p = 0.03).

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Fig. 3: Strain in the lateral ulnar collateral ligament versus relative cubitus varus deformity as assessed with repeated-measures analysis of variance. The strain increased as the degree of varus increased (r = 0.429). At 30° of cubitus varus deformity, there was a significant increase in ligament strain compared with that in the native elbow. *p < 0.05.

Analysis of covariance to correct for size differences among the specimens also showed a significant increase in the strain in the lateral ulnar collateral ligament, at a supination moment of 1.2 Nm, as the varus deformity increased (Fig. 4). However, whereas the repeated-measures analysis of variance showed a significant increase only at 30° of varus deformity, the analysis of covariance revealed a significant increase at 25° of cubitus varus as well (Fig. 4).

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Fig. 4: Strain in the lateral ulnar collateral ligament versus relative cubitus varus deformity as assessed with repeated-measures analysis of covariance with the transepicondylar width of the specimen used as the covariate. At 25° and 30° of cubitus varus deformity, the ligament strain was found to be significantly increased compared with that in the native elbow. *p < 0.05.

Ulnohumeral joint-widening was found to increase, at a supination moment of 1.2 Nm, as the cubitus varus deformity increased (Fig. 5, r = 0.628, p < 0.001). The slope of the curve remained near zero until 15°, which was the inflection point after which the slope began to rise. Ulnohumeral joint-widening increased significantly, compared with the value for the undisturbed native elbow, at 25° of cubitus varus deformity and remained significantly increased at 30°.

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Fig. 5: Ulnohumeral joint-widening versus relative cubitus varus deformity as assessed with repeated-measures analysis of variance. The ulnohumeral joint subluxation increased as the degree of varus increased (r = 0.628). At 25° and 30° of cubitus varus deformity, joint-widening was found to be significantly increased compared with that in the native elbow. *p < 0.05.

Analysis of covariance to correct for size differences among the specimens showed ulnohumeral joint-widening to increase, at a supination moment of 1.2 Nm, as the varus deformity increased (Fig. 6). Again, the slope of the curve remained near zero until the inflection point of 15°, after which the slope began to rise. However, in contrast with the results of the repeated-measures analysis of variance, which showed a significant increase at 25° of cubitus varus, the analysis of covariance showed a significant increase at 20° of cubitus varus (Fig. 6).

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Fig. 6: Ulnohumeral joint-widening versus relative cubitus varus deformity as assessed with repeated-measures analysis of covariance with the transepicondylar width of the specimen as the covariate. At 20°, 25°, and 30° of cubitus varus deformity, joint-widening was found to be significantly increased compared with that in the native elbow. *p < 0.05.

Discussion

Materials and Methods | Results | Discussion | References

Recently, posterolateral rotatory instability of the elbow was described in a group of patients at a mean of twenty-seven years following the development of cubitus varus deformity of the ipsilateral elbow⁴. The mechanism for this "tardy" instability was thought to be changes in elbow biomechanics resulting in increased strain on the lateral ulnar collateral ligament, eventually leading to insufficiency of the ligament. The results of our study support the theory that cubitus varus deformity results in increased strain in the lateral ulnar collateral ligament. Furthermore, even with an intact lateral ulnar collateral ligament, the ulnohumeral joint was found to widen or open with increasing cubitus varus deformity. The fact that ulnohumeral joint-widening was achievable in an elbow with an intact lateral ulnar collateral ligament is interesting in that it suggests that the lateral ulnar collateral ligament is not sufficient by itself to prevent subluxation when the cubitus varus increases beyond a threshold of 25° (20° when corrected for specimen size). This is likely a result of the loss of the other soft-tissue constraints about the elbow, such as the common extensor origin, which have been shown to contribute to rotatory stability of the elbow¹⁴.

Previously, cubitus varus deformity of the elbow was thought to be only a cosmetic deformity with few long-term sequelae^18-20. However, it appears, on the basis of this biomechanical study, that the deformity subjects the lateral ulnar collateral ligament to increased strain and results in ulnohumeral joint-widening or subluxation. With time, this may be associated with attenuation of the lateral ulnar collateral ligament and the development of clinically relevant posterolateral rotatory instability of the elbow. The findings of this research study have clinical relevance in three areas. First, they suggest a relationship between cubitus varus and strain in the lateral ulnar collateral ligament and suggest a mechanism for the development of posterolateral rotatory instability of the elbow in such individuals. Second, the findings suggest an amount of varus angulation (20° to 25°) that may be unacceptable on a biomechanical level in the primary treatment of distal humeral fractures. Third, the findings strongly support the inclusion of a corrective distal humeral osteotomy in a surgical reconstruction performed to treat posterolateral rotatory instability of the elbow. Without such an osteotomy, a surgically repaired ligament may be subjected to increased strains comparable with those that led to the original failure of the lateral ulnar collateral ligament.

In their clinical study of twenty-five elbows with cubitus varus deformity in which clinically relevant posterolateral rotatory instability of the elbow eventually developed, O'Driscoll et al.⁴ found the average varus deformity to be 15° (range, 0° to 35°). However, they measured absolute varus deformity at the elbow, whereas our technique measured cubitus varus relative to an initial carrying angle of the elbow. To directly compare the results of our study with those of O'Driscoll et al., we need to correct for the carrying angle. In our study, the measured carrying angle averaged 5.1° ± 1.4°, so the absolute varus deformity at the elbow was actually about 5° less than our reported relative varus deformity. When corrected for the carrying angle, the strain in the lateral ulnar collateral ligament became significantly increased at about 25° of cubitus varus (or 20° when specimen size was accounted for). In comparison, ulnohumeral joint-widening became significantly increased at about 20° (or 15° when specimen size was accounted for). However, rather than identifying a particular angular deformity at which an unacceptable increase in strain occurs, our findings may represent a gradual increase (with the inflection point of the curve in the 15° to 20° range) that results in clinical symptoms, depending on other variables such as the duration of deformity, activity level, and dynamic stabilizers. In this regard, our findings are consistent with those of prior clinical studies and may also explain the range of deformity seen in those patients (0° to 35°)¹.

Our study had limitations. Testing was performed on cadaveric specimens, from geriatric donors, that had been stripped of all soft tissues except the joint capsule and ligaments. As a result, the elbows were devoid of all dynamic stabilizers. This may be important since muscular attachments have been shown to have some role in maintaining elbow stability¹⁴. Furthermore, the loads applied to the elbow were not physiologic because, with no other soft tissues to attenuate the load, the ligaments were subjected to all of the loading. However, the fact that our loads were adequate to achieve strain in the ligament without leading to ligament failure suggests that they were appropriate. An in vivo study in which a remote implantable strain gauge collected data during daily activities would have allowed more physiologic conditions to be reproduced, but this technology is currently available only for intra-articular ligaments such as the anterior cruciate ligament^21,22. The fact that our specimens were obtained from a geriatric population may have resulted in the soft tissues being atrophied relative to that in young individuals with clinical posterolateral rotatory instability of the elbow. Ligaments from geriatric populations have been shown to have decreased collagen fibril diameter, modulus of elasticity, failure stress, and strain energy^23-25. Thus, although our sample population was consistent, the stress-versus-strain relationship for the ligament probably differs from that in a younger population. Also, we cannot prove that the increased strain documented in the lateral ulnar collateral ligaments in our study will necessarily result in clinical symptoms years or decades later. Finally, malunion following supracondylar humeral fracture is a complex three-dimensional deformity that includes varus, extension, and rotation elements. With the limitations of our model, we examined only the deformity that we thought to be most important (varus). It may be that our findings would change with inclusion of other planes of malalignment.

In conclusion, in this in vitro cadaveric study, cubitus varus deformity of the elbow resulted in increased strain in the lateral ulnar collateral ligament, with a corresponding increase in ulnohumeral joint-opening consistent with the posterolateral rotatory instability of the elbow seen clinically. Increasing degrees of deformity resulted in increases in strain and joint-opening, which were significant between 15° and 20° of deformity. We believe that these findings have implications for the etiology, prevention, and treatment of posterolateral rotatory instability of the elbow that occurs following cubitus varus deformity.

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Topics

elbow region ; humerus ; ligaments ; supination ; elbow instability ; ulnar collateral ligament ; gunstock deformity ; humeroulnar joint ; varus deformity

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Murray J. Beuerlein, Jeffrey T. Reid, Emil H. Schemitsch, Michael D. McKee; Effect of Distal Humeral Varus Deformity on Strain in the Lateral Ulnar Collateral Ligament and Ulnohumeral Joint Stability. The Journal of Bone & Joint Surgery. 2004 Oct;86(10):2235-2242.

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