Is Ulnar Collateral Ligament Strength Proportional to Height and Weight?

Methods

Cadaveric Specimens

Twenty unmatched fresh-frozen cadaveric right upper extremities from adult male donors 40 years of age or younger were procured and allowed to thaw to room temperature for approximately 24 hours before the beginning of the experiment. Our institution did not require institutional review board review or approval for cadaveric studies. This age range was intended to be representative of the throwing athlete population of interest. Because roughly 90% of the general population is right-handed, ²⁶ most right arms can be assumed to be dominant. Hence, only right arms were selected to reduce any effects of arm dominance. Donor height and weight were provided by the vendor (Science Care) and documented for later analysis (Table 1). Donor causes of death were noted to be largely (17/20) sudden and/or traumatic (ie, cardiac arrest, drug toxicity, blunt force, and gunshot). The remaining causes of death (3/20) were due to seemingly prolonged illness/disease (eg, diabetes, liver failure, and enlarged heart).

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Table 1

Specimen Characteristics ^a

Donor	Age, y	Height, m	Mass, kg	BMI, kg/m2
F201403	37	1.80	90.7	27.9
S221856	28	1.73	61.2	20.5
L221441	38	1.85	89.8	26.1
I230078	36	1.73	43.5	14.6
C230170	35	1.91	90.7	25.0
C230212	38	1.80	112.9	34.7
C230242	34	1.83	88.0	26.3
C230344	22	1.78	81.6	25.8
L230273	28	1.83	109.3	32.7
L230360	38	1.73	86.6	29.0
S230265	34	1.80	69.9	21.5
P230239	30	1.80	100.2	30.8
F230212	32	1.80	59.0	18.1
F230277	34	1.80	104.3	32.1
L230891	39	1.73	78.5	26.3
F230817	35	1.78	78.5	24.8
F230833	38	1.78	81.6	25.8
L231118	19	1.83	61.7	18.4
C231261	31	1.70	95.3	32.9
L231097	40	1.91	82.6	22.7
Mean ± SD	33 ± 6	1.80 ± 0.06	83.3 ± 17.7	25.8 ± 5.4

a

BMI, body mass index.

Geometric Measurements

Before dissection, geometric measurements of the UCL were made under direct sonographic visualization. A Samsung HS60 ultrasound machine with an LA4-18BD linear probe (Samsung/Neuro Logica Corp) was used by a sports medicine physician with 13 years of experience with musculoskeletal ultrasound (R.E.C.) to noninvasively assess UCL thickness and length (Figure 1). Thickness was defined as a line beginning from the lateral/deep origin of the medial epicondyle and ending at a point perpendicular to the overall medial/superficial length, similar to a previous study. ²⁰ This was later used for calculation of cross-sectional area. Sonographic measurements were recorded with the specimen’s elbow stabilized at 90° of flexion to ensure repeatability.

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Figure 1.

Ultrasound image of a right elbow. The 5 measurements illustrated are (1) length of the medial epicondyle origin, (2) medial-side length from origin to insertion, (3) distance between medial and lateral insertion points on the sublime tubercle, (4) lateral-side length from origin to insertion, and (5) ulnar collateral ligament thickness.

Biomechanical Testing

Once initial dissections were completed, the humerus and forearm were potted in an acrylic pipe using polymethylmethacrylate. Each specimen was then secured into a custom test fixture that we have used extensively^21,28 and mounted into an MTS 858 MiniBionix II servohydraulic mechanical test frame (MTS Systems) at a fixed elbow flexion angle of 90° (Figure 2). Biomechanical testing in the intact condition was conducted by commanding the test frame actuator in torque control to internally rotate the humerus while holding the forearm stationary, creating a valgus torque on the medial elbow. A 2-N·m valgus preload was applied, followed by 10 cycles of valgus joint torque between 2 and 10 N·m to provide a consistent strain history as well as to precondition the soft tissues. This range was chosen based on a previous study, ¹² to fall well within the physiological (ie, elastic) range of the joint. At the completion of these subfailure cycles, a return-to-zero command was followed by a constant torque-to-failure test in rotation control at a rate of 1 deg/s as in previous studies.^21,28 Failure was defined as both the sudden appearance of a visible defect in the tissue and a concurrent dramatic decrease in the real-time torque-rotation curve. Failure torque was defined as the torque at the instant of failure. Stiffness was calculated as the slope of the linear region of the torque-rotation curve.

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Figure 2.

Biomechanical test setup with a right elbow joint fixed at 90° of flexion. The humerus is attached to the test frame actuator, which rotates to create a valgus torque in the elbow, measured by the torque cell attached to the ulna/radius.

Results

Failure Modes

Of the 20 specimens tested under valgus torsion, 18 failed at the UCL, while 2 specimens failed via humeral fracture. Failure strength data were not used for the latter; however, their native geometric properties (ie, cross-sectional area) and subfailure mechanical properties (ie, stiffness) were used for analysis. Of the 18 UCL failures, 7 (39%) failed near the medial epicondyle, 6 (33%) failed near the sublime tubercle, and 5 (28%) failed in the midsubstance (Figure 3).

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Figure 3.

Midsubstance failure of a right ulnar collateral ligament (UCL) subjected to a constant elbow valgus rotation.

Biomechanical Properties

UCL failure torque (n = 18) resulting from valgus rotation of the elbow at 90° of flexion was 45.0 ± 10.5 N·m (Table 2). Stiffness (n = 20) during the torque-to-failure test was 2.72 ± 0.48 N·m/deg. Linear regression showed no significant correlation (P≥ .25; R≤ 0.28) between failure torque and height, height × weight, weight, or body mass index (BMI). Similarly, stiffness during the torque-to-failure test showed no significant correlation (P≥ .18; R≤ 0.31) with height, height × weight, weight, or BMI.

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Table 2

UCL Parameters ^a

	Mean ± SD	vs Height		vs Weight		vs Height × Weight		vs BMI
		P	R	P	R	P	R	P	R
Cross-sectional area, mm2	39.1 ± 11.2	.34	0.23	.065	0.42	.064	0.42	.085	0.39
Length, mm	21.8 ± 2.3	.51	0.16	.22	0.29	.21	0.29	.26	0.27
Stiffness, N·m/deg	2.72 ± 0.48	.24	0.28	.21	0.29	.18	0.31	.35	0.22
Failure torque, N·m	45.0 ± 10.5	.25	0.28	.85	0.05	.72	0.09	.85	−0.04

a

BMI, body mass index; UCL, ulnar collateral ligament.

Discussion

Contrary to our hypothesis, native UCL cross-sectional area did not significantly correlate with donor height, weight, height × weight, or BMI. This finding is in contrast to findings from recent clinical studies, which positively correlated patient height with various tendon diameters.^3,24,31 It is conceivable that differences between tendons and ligaments could account for this discrepancy; however, a more likely explanation is that the previous studies used tendons from the lower extremities, which may have been developmentally affected by body weight inherently more than the upper extremity, nonweightbearing ligament in the present study.

Furthermore, our hypothesis that UCL strength and stiffness are correlated with body size was not supported by the data. To our knowledge, there have been no previous studies investigating in vitro failure torque of the elbow as it relates to the donor’s anthropometry. Previous experiments have shown failure torques ranging from 19 to 34 N·m for the native UCL.^1,18,32 Our mean failure torque of 45 N·m was higher than this range, possibly because of differences in test factors between studies such as magnitude of elbow flexion during valgus loading and rate of loading. Additionally, the inclusion of only male specimens and right arms and the relatively young age of specimen donors in our study likely contributed to the greater failure torque experienced compared with previous work.

Because UCL failure torque and stiffness were not correlated with body weight and height, we recommend that biomechanical analyses of adult baseball pitchers should report absolute magnitudes of elbow varus torque instead of values normalized by body weight and height. Several recent publications have focused on absolute magnitude of elbow varus torque. For example, Escamilla et al ¹⁴ compared the biomechanics of professional pitchers with high, moderate, and low contralateral trunk tilt at the instant of ball release and found no difference in absolute magnitude of elbow varus torque among the 3 groups. Solomito et al ³³ divided the biomechanics of 99 collegiate pitchers into quartiles based on their stride length. There was lower, but not significantly different, elbow varus torque in the 2 middle groups (70.3 ± 15.9 N·m and 73.4 ± 15.5 N·m) compared with the shortest stride group (78.3 ± 11.8 N·m) and longest stride group (79.5 ± 15.8 N·m). Dowling et al ¹¹ compared high-velocity and low-velocity professional pitchers and reported greater elbow varus torque in the high-velocity group (91 ± 15 N·m) than the low-velocity group (84 ± 15 N·m). Studies such as these, in which elbow varus torque values are reported in newton-meters (instead of unitless normalized values), provide better insight into biomechanics that may lead to UCL stress and risk of failure.

The absence of a correlation between UCL strength and body size suggests that the sports medicine physician can maintain a consistent approach to treatment of adult baseball pitchers, with no special adjustments to clinical evaluation and/or surgical treatment being made based on their physical stature. In the context of the preoperative clinical examination by the surgeon, our findings could help guide the discussion with the patient regarding expected surgical techniques and treatments. Our data imply that factors beyond mere size, such as pitching mechanics, training regimen, and individual physiological differences, may play more significant roles in determining UCL strength and injury risk. Therefore, the physician’s focus should be on treating patients based solely on clinical indications, while assessing the multifaceted elements comprehensively to tailor treatment plans effectively for all pitchers, regardless of their body size, ensuring that optimal care and rehabilitation strategies are used to promote long-term health and performance.

In the realm of physical therapy, Reinold and colleagues ²⁷ recently published a new interval throwing program based on biomechanical data as well as clinical experience with today’s baseball pitchers. This updated program replaces their longstanding interval throwing program published years ago. In developing the new program, the authors considered the workload of flat ground throws and partial effort pitches prescribed, based on the absolute elbow varus torque (N·m) of each throw, not normalized torque. The fact that the current study found no relationship between UCL strength and body size supports that development decision and suggests that the new interval throwing program can be utilized by adult baseball pitchers of all heights and weights.

Conclusion

Based on our study, valgus failure torque in the elbow does not correlate with patient height or weight. This would suggest that elbow torque from biomechanical analysis of baseball pitchers and other athletes should not be normalized by size but rather be reported as absolute kinetic measurements. The results from this study have the potential to improve the clinical value of studies that investigate pitching biomechanics and their effect on elbow loading as well as also providing valuable insight for surgical and rehabilitative decisions in the treatment of elbow injuries.