The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 86, Issue 3

Scientific Articles | March 01, 2004

Influence of Humeral Prosthesis Height on Biomechanics of Glenohumeral Abduction: An In Vitro Study

Richard W. Nyffeler, MD¹; Ralph Sheikh, MD²; Hilaire A.C. Jacob, PhD²; Christian Gerber, MD²

¹ Orthopaedic Hospital, University of Lausanne, Avenue Pierre-Decker 4, 1005 Lausanne, Switzerland. E-mail address: richard.nyffeler@bluewin.ch
² Department of Orthopaedic Surgery, University of Zurich, Balgrist, Forchstrasse 340, 8008 Zurich, Switzerland

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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 the Department of Orthopaedic Surgery, University of Zurich, Balgrist, Zurich, Switzerland

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2004 Mar 01;86(3):575-580

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Abstract

Background: During shoulder replacement surgery, the normal height of the proximal part of the humerus relative to the tuberosities frequently is not restored because of differences in prosthetic geometry or problems with surgical technique. The purpose of the present study was to determine the effect of humeral prosthesis height on range of motion and on the moment arms of the rotator cuff muscles during glenohumeral abduction.

Methods: Tendon excursions and abduction angles were recorded simultaneously in six cadaveric specimens during passive glenohumeral abduction in the scapular plane. Moment arms were calculated for each muscle by computing the slope of the tendon excursion-versus-glenohumeral abduction angle relationship. The experiments were carried out with the intact joint and after replacement of the humeral head with a prosthesis that was inserted in an anatomically correct position as well as 5 and 10 mm too high.

Results: Insertion of the prosthesis in positions that were 5 and 10 mm too high resulted in significant and marked reductions of the maximum abduction angle of 10° (range, 5° to 18°) and 16° (range, 12° to 20°), respectively. In addition, the moment arms of the infraspinatus and subscapularis decreased by 4 to 10 mm. This corresponded to a 20% to 50% decrease of the abduction moment arms of the infraspinatus and an approximately 50% to 100% decrease of the abduction moment arms of the subscapularis, depending on the abduction angle and the part of the muscle being considered.

Conclusions: If a humeral head prosthesis is placed too high relative to the tuberosities, shoulder function is impaired by two potential mechanisms: (1) the inferior capsule becomes tight at lower abduction angles and limits abduction, and (2) the center of rotation is displaced upward in relation to the line of action of the rotator cuff muscles, resulting in smaller moment arms and decreased abduction moments of the respective muscles.

Clinical Relevance: In patients managed with shoulder replacement surgery, limitation of range of motion, loss of abduction strength, and overload with long-term failure of the supraspinatus tendon are potential consequences of positioning the humeral head of the prosthesis proximal to the anatomic position.

Figures in this Article

Shoulder arthroplasty is known to be an effective method for restoring function and reducing pain in patients with degenerative shoulder joint disease or severely comminuted fractures of the humeral head. The outcome of partial or total shoulder joint replacement depends on the pathology, the surgical technique, and the rehabilitation program^1,2. Recent reports have suggested that failure might be due to technical errors, such as component malpositioning^3-5, or to the use of a nonanatomic prosthesis design⁶. Pearl and Kurutz⁶ showed that a commonly used press-fit prosthesis displaced the center of rotation superiorly by a mean of 12 mm and laterally by a mean of 8 mm. They believed that elevation of the center of rotation can result in rotator cuff tendinopathy and superior head migration and thereby predispose the glenoid component to loosening. However, to our knowledge, the influence of elevation of the center of rotation on kinetics and kinematics has not been investigated experimentally. The purpose of the present study was to determine the effect of superior displacement of the center of rotation on the passive range of motion and on the moment arms of the rotator cuff during glenohumeral abduction in the scapular plane.

Materials and Methods

Materials and Methods | Results | Discussion | References

Six fresh-frozen shoulder specimens from six cadavers were obtained from the Institute of Pathology of our university. No medical histories were available. All shoulders were harvested by disarticulation at the thoracic wall and transection at the middle of the humeral shaft. Four left shoulders and two right shoulders were used. None of the specimens showed signs of trauma, previous operations, rotator-cuff tears, or degenerative joint disease.

Dissection

The overlying muscles were dissected, leaving the glenohumeral joint and the rotator cuff intact. The muscles of the rotator cuff were then replaced with cords in the following manner. The subscapularis and infraspinatus were divided into three portions each according to the width of the musculotendinous units. The supraspinatus was divided into an anterior and a posterior portion. The teres minor was not subdivided. The divided rotator cuff muscles were then elevated from their origins at the scapular blade, and eye-hooks were placed close to the scapular neck to simulate the centroids of the different muscle portions. The muscles were transected proximal to their musculotendinous junction and were replaced with cords representing the line of action of each muscle portion. These cords were attached to the corresponding tendinous insertions on the humeral head with use of a number-3 Ethibond suture and a modified Mason-Allen stitch⁷ and were pulled through the corresponding eye-hooks on the scapular blade. The subdivision of the rotator cuff muscles allowed for simulation of the upper, middle, and lower parts of the subscapularis and infraspinatus muscles, in contrast with previous studies in which these muscles were simulated with a single wire and an origin close to the medial border of the scapula⁸. The scapula was fixed rigidly to a frame, with the glenoid surface oriented vertically. A weight of 2 N was attached to the freely hanging end of each cord to take up the slack and to center the humeral head in the glenoid cavity (Fig. 1). Linear voltage displacement transducers with strain gauges (Hottinger Baldwin Messtechnik, Darmstadt, Germany) were connected to the cords to measure the tendon excursions. The receiver of an electromagnetic tracking device for angular measurements (Flock of Birds; Ascension Technology, Burlington, Vermont) was mounted to an aluminum rod along the axis of the humeral shaft (Fig. 1). The transmitter was placed close to the scapula with the x and y axes parallel to the plane of the scapula. The measurement accuracies of the devices used to assess tendon excursions and angular rotations were 0.1 mm and 2°, respectively⁹.

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Fig. 1: Illustration of the experimental setup. The scapula was fixed vertically to a frame. The rotator cuff was divided into nine portions (two supraspinatus portions, three infraspinatus portions, three subscapularis portions, and one teres minor portion) and was replaced with cords representing the lines of action of the different muscle portions. Abduction angles were measured with an electromagnetic tracking device, with the transmitter (T) fixed to the scapula and the receiver (R) fixed to the humerus, and tendon excursions were recorded with linear voltage displacement transducers (LVDTs) connected to the cords.

Measurement Procedure

The humerus was passively elevated in neutral rotation from adduction to maximal glenohumeral abduction in the scapular plane. Neutral rotation was defined as the position lying in the middle of maximum external and internal rotation. Elevation was achieved with light assistance by the examiner's hand and was repeated three times for every experiment. During this movement, the abduction angles and the tendon excursions were recorded simultaneously at a frequency of 10 Hz with use of a personal computer equipped with TestPoint (Keithley Instruments, Germering, Germany) and Excel (Microsoft, Redmond, Washington) software. After the measurements were done for the intact joint, two 2.5-mm holes were drilled into the lesser tuberosity, an osteotomy of the lesser tuberosity was performed, and the rotator interval was incised to allow exposure of the articular surfaces. The humeral head was resected at the level of the anatomical neck, and a Neer II prosthesis (Biomet, Warsaw, Indiana) was inserted until it was in full contact with the osteotomy surface (anatomical height). The glenoid was not replaced. The lesser tuberosity was reattached in the anatomical position with two Kirschner wires that were inserted into the predrilled holes, and the rotator interval was closed with two stitches. Passive elevation was then repeated, and the abduction angles and tendon excursions were recorded. The joint was then reopened, and the prosthesis was elevated by 5 mm along the axis of the stem without affecting the version angle of the prosthetic head. This resulted in an isolated elevation of the center of rotation by the same distance without changing the offset. A spacer was inserted between the prosthesis and the osteotomy surface to maintain prosthetic height. The joint was closed in the fashion described above, and the movement and recordings were repeated. Finally, the experiment was repeated again with the prosthesis elevated by 10 mm. Under each test condition, three consecutive cycles of abduction and adduction were performed without removal or repositioning of the specimen.

Analysis of the Data

The reproducibility of the tendon excursion and abduction angle measurements was evaluated by computing the coefficients of correlation (R) for the three trials of each experiment. Polynomial regression was used to model the relationship between tendon excursion (in millimeters) and abduction angle (in radians). Because this relationship was typically s-shaped or parabolic, a third-order polynomial was selected to fit the experimental data (Fig. 2). The slope of each polynomial was computed by means of numerical differentiation and corresponded to the lever arm length of the respective muscle portion^8,10 (Fig. 3). A repeated-measures analysis of variance with a post hoc Bonferroni Dunn test was used to determine if changes in the prosthetic height resulted in significant differences in the moment arms. The level of significance was set at p = 0.05. Because range of motion varied between specimens and decreased with elevation of the prosthesis, statistical analysis could only be done for angles that were common among all specimens and treatment groups. Statistical analysis was therefore conducted for 20°, 40°, and 60° of glenohumeral abduction.

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Fig. 2: Illustration of the typical tendon excursionabduction angle relationship for a single muscle portion after three trials. A thirdorder polynomial was fitted to the measured values. The slope of the curve was computed by means of numerical differentiation and corresponded to the lever arm length of the respective muscle portion.

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Fig. 3: Illustration showing the relationship between the change of angle (da), the corresponding tendon excursion (ds), and lever arm length (a) during abduction in the scapular plane. An electromagnetic tracking device was used to record angular measurements, and linear voltage displacement transducers (LVDT) were used to record tendon excursions. T = transmitter, and R = receiver.

Results

Materials and Methods | Results | Discussion | References

The tendon excursion and abduction angle measurements were found to be reproducible. The coefficients of correlation for the three consecutive trials of each experiment were always >0.95. In the intact joint, the supraspinatus, the infraspinatus, and the superior two-thirds of the subscapularis shortened during abduction and therefore acted as agonists. The inferior part of the subscapularis showed almost no excursion. The teres minor elongated during abduction and therefore acted as an antagonist. The calculated moment arms changed continuously in a nonlinear way throughout the arc of motion (Fig. 4). The insertion of the infraspinatus was found to be more cranial and lateral than that of the subscapularis. Accordingly, the moment arms of the infraspinatus were 5 to 10 mm larger than those of the subscapularis during abduction in the scapular plane. The greatest moment arm values were measured for the supraspinatus. There was no significant difference between the lever arms of the anterior and posterior portions of the supraspinatus.

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Fig. 4: Illustrations showing the average moment arms in relation to the glenohumeral abduction angle and the prosthetic height for the different muscle portions tested. Because there were no significant differences between the anterior and posterior parts of the supraspinatus, the average moment arm for the two parts is reported. Positive moment arms correspond with agonists, and negative moment arms correspond with antagonists. Intact = intact specimen, P + 0 = prosthesis at anatomical height, P + 5 = prosthesis elevated 5 mm, and P + 10 = prosthesis elevated 10 mm.

When the Neer prosthesis was inserted anatomically, the lengths of the moment arms were similar to those of the intact specimen (Fig. 4). Elevation of the prosthesis by 5 and 10 mm was accompanied by a decrease of the moment arms of the infraspinatus and the subscapularis by 4 to 10 mm, depending on arm position and the muscle part considered (Fig. 4). For these two muscles, the decrease of the moment arms was significant at 40° and 60° of abduction when the prosthesis was positioned 10 mm higher than normal (p < 0.05). This corresponded to a relative decrease of the moment arms of approximately 20% to 50% for the entire infraspinatus and approximately 50% to 100% for the entire subscapularis. The moment arm of the supraspinatus was slightly but not significantly decreased when the prosthesis was elevated 5 and 10 mm. The moment arm of the teres minor was significantly increased when the prosthesis was elevated 5 and 10 mm (p < 0.05). Furthermore, the maximal abduction angle decreased by 10° (range, 5° to 18°) when the prosthesis was elevated 5 mm and by 16° (range, 12° to 20°) when the prosthesis was elevated 10 mm (Fig. 5). These decreases in abduction were significant (p < 0.05).

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Fig. 5: Illustration showing the maximal abduction angle as a function of prosthetic height. The values are expressed as the mean and the standard deviation following tests of the intact joint, tests with the Neer-II prosthesis at anatomical height, and tests with the prosthesis elevated 5 mm and 10 mm. Significant differences compared with the intact joint are marked with an asterisk.

Discussion

Materials and Methods | Results | Discussion | References

It is thought that restoring the normal anatomy of the humeral head by means of a hemiarthroplasty provides the best functional results. In a recent study, Pearl and Kurutz⁶ showed that commonly used second-generation press-fit shoulder prostheses (Bio-Modular total shoulder system [Biomet], Global total shoulder arthroplasty system [DePuy, Warsaw, Indiana], Select shoulder system [Intermedics Orthopedics, Austin, Texas], Kirschner II-C shoulder system [Biomet]) did not restore the original joint geometry precisely but displaced the center of rotation superiorly and laterally. The authors suggested that these alterations of joint geometry predispose the joint to complications following shoulder arthroplasty. This hypothesis was supported by Moskal et al.⁴, who analyzed 122 failed shoulder arthroplasties and found that the humeral head component was placed too high in 60% of the cases.

The purpose of the present study was to evaluate the kinematic and kinetic effects of superior elevation of the center of rotation as commonly observed in clinical practice.

In the intact specimens, all muscle fibers of the supraspinatus, the infraspinatus, and the upper two-thirds of the subscapularis shortened during abduction in the scapular plane (Fig. 4). These findings are in accordance with the observations of other authors^8,10-13. Because abduction strength is a product of muscle activity, cross-sectional area, and moment arm¹³, it must be concluded that these muscles are not only stabilizers¹⁴ but also abductors of the normal joint.

Tendon excursions and moment arms depend on the position of the tendon fibers relative to the instantaneous center of rotation. Therefore, changing either the center of rotation or the height of the tendinous insertions may influence the kinetics of the joint. In our experiments, the humeral prosthesis was elevated 5 and 10 mm, simulating a rise of the center of rotation relative to the tuberosities by the same amount. This resulted in a significant reduction of the moment arms of the rotator cuff muscles. Because the moment arms of these muscles are small, they experience an important relative change in association with small variations in the height of the center of rotation and therefore sufficient elevation of the center of rotation can transform an abductor into an adductor (Fig. 6)!

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Fig. 6: Illustration of the influence of the height of the humeral prosthesis on the function of the rotator cuff muscles. If the center of rotation (CR, indicated by the small circle) is elevated after insertion of a shoulder prosthesis, the infraspinatus and subscapularis (SSC) muscles may change from abductors (with the tendon fibers proximal to the center of rotation) to adductors (with the tendon fibers distal to the center of rotation). SSP = supraspinatus.

Insertion of a shoulder prosthesis at an abnormal height also resulted in significant reduction of the maximal glenohumeral abduction angle. This finding was due to tensioning of the inferior capsuloligamentous structures in abduction and not to mechanical impingement. In fact, elevation of the prosthesis relative to the humeral shaft corresponds with a lowering of the medial cortex of the proximal part of the humerus relative to the glenoid, which results in tensioning of the inferior capsule. In patients with proximal humeral fractures, similar reductions in lever arm length and abduction angles can be expected when the prosthesis is implanted at the correct height but the tuberosities are overreduced distally. This also might be one explanation for unsatisfactory functional results after a hemiarthroplasty is performed for the treatment of a fracture.

Loss of abduction, as is observed when the head is located too proximally, may explain a failure to regain normal active overhead function. Moreover, this limitation must be expected to lead to eccentric loading of the glenoid with attempted abduction and could result in glenoid loosening.

The present study has several limitations: the tests were performed in a cadaver model, the abduction movements were done passively, and the abduction movements were limited to the plane of the scapula. In this in vitro study, tendons and muscles were simulated by wires that passed through the centroids of the corresponding cross-sectional areas and represented the lines of action. The definition of such lines of action is more straightforward for long, slim muscles and is more difficult for muscles with broad origins, such as the subscapularis and infraspinatus. As changing either the origin or the insertion of a muscle action line affects the tendon excursions and lever arm lengths, special attention was paid to the preparation of the specimens. Passive motion was preferred to active motion because the load distribution within the different muscle portions of the rotator cuff is not known. Instead of simulating muscle function by applying hypothetical loads to the different muscle groups, the humeral head was centered on the glenoid cavity by small loads of 2 N that were applied to each of the nine lines of action of the rotator cuff and abduction was achieved with light assistance by the examiner's hand. This method was found to be reproducible as shown by the coefficients of correlation (R > 0.95) between the three trials of each experiment. Another limitation of the study is that the order of the prosthetic positions during testing was not randomized. The use of a spacer between the prosthetic head and the osteotomy surface and strict avoidance of overstretching the joint capsule during abduction movements were effected to minimize systematic errors.

In conclusion, elevation of the center of rotation may impair shoulder function through two basic mechanisms. First, the inferior capsuloligamentous structures become tight even at low abduction angles and therefore limit abduction, especially if they are not adequately released during surgery. Second, the moment arms of the agonists decrease and those of the antagonists increase, resulting in an overall reduction of the abduction moment of the entire rotator cuff.Our findings suggest that particular attention must be paid to correct placement of the prosthesis and tuberosity during proximal humeral replacement and that loss of scapular plane abduction and abductor strength may be direct consequences of implantation of a humeral prosthesis at an improper height.

References

Materials and Methods | Results | Discussion | References

Boileau P, Caligaris-Cordero B, Payeur F, Tinsi L, Argenson C. [Prognostic factors during rehabilitation after shoulder prostheses for fracture]. Rev Chir Orthop Reparatrice Appar Mot.1999;85: 106-16. French.85106 1999

Iannotti JP, Williams GR. Total shoulder arthroplasty. Factors influencing prosthetic design. Orthop Clin North Am.1998;29: 377-91.29377 1998 [PubMed][CrossRef]

Mohammed KD, Sonnabend DH. Revision shoulder arthroplasty [abstract]. J Shoulder Elbow Surg.1999;8: 553.8553 1999

Moskal MJ, Duckworth D, Matsen FA. An analysis of 122 failed shoulder arthroplasties [abstract]. J Shoulder Elbow Surg.1999;8: 554.8554 1999

Compito CA, Self EB, Bigliani LU. Arthroplasty and acute shoulder trauma. Reasons for success and failure. Clin Orthop.1994;307: 27-36.30727 1994 [PubMed]

Pearl ML, Kurutz S. Geometric analysis of commonly used prosthetic systems for proximal humeral replacement. J Bone Joint Surg Am.1999; 81: 660-71.81660 1999 [PubMed][CrossRef]

Gerber C, Schneeberger AG, Beck M, Schlegel U. Mechanical strength of repairs of the rotator cuff. J Bone Joint Surg Br.1994;76: 371-80.76371 1994 [PubMed]

Liu J, Hughes RE, Smutz WP, Niebur G, Nan-An K. Roles of deltoid and rotator cuff muscles in shoulder elevation. Clin Biomech (Bristol, Avon).1997;12: 32-8.1232 1997 [PubMed][CrossRef]

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Otis JC, Jiang CC, Wickiewicz TL, Peterson MG, Warren RF, Santner TJ. Changes in the moment arms of the rotator cuff and deltoid muscles with abduction and rotation. J Bone Joint Surg Am.1994;76: 667-76.76667 1994

Sharkey NA, Marder RA, Hanson PB. The entire rotator cuff contributes to elevation of the arm. J Orthop Res.1994;12: 699-708.12699 1994 [CrossRef]

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Topics

biomechanics ; humerus ; prostheses and implants ; prosthesis implantation ; rotator cuff ; tendon ; arm ; supraspinatus ; infraspinatous muscle structure ; abduction

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Richard W. Nyffeler, Ralph Sheikh, Hilaire A.C. Jacob, Christian Gerber; Influence of Humeral Prosthesis Height on Biomechanics of Glenohumeral AbductionAn In Vitro Study. The Journal of Bone & Joint Surgery. 2004 Mar;86(3):575-580.

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