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Biomechanical analysis of intramedullary vs. superior plate fixation of transverse midshaft clavicle fractures
ARTICLE IN PRESS J Shoulder Elbow Surg (2015) ■■, ■■–■■
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Biomechanical analysis of intramedullary vs. superior plate fixation of transverse midshaft clavicle fractures David J. Wilson, MDa,*, William F. Scully, MDb, Kyong S. Min, MDc, Tess A. Harmon, BAd, Josef K. Eichinger, MDa, Edward D. Arrington, MDa a
Madigan Army Medical Center, Joint Base Lewis McChord, WA, USA Martin Army Community Hospital, Ft. Benning, GA, USA c Brian Allgood Army Community Hospital, Yongsan, South Korea d Loma Linda University School of Medicine, Loma Linda, CA, USA b
Background: Middle-third clavicle fractures represent 2% to 4% of all skeletal trauma in the United States. Treatment options include intramedullary (IM) as well as plate and screw (PS) constructs. The purpose of this study was to analyze the biomechanical stability of a specific IM system compared with nonlocking PS fixation under low-threshold physiologic load. Methods: Twenty fourth-generation Sawbones (Pacific Research Laboratories, Vashon, WA, USA) with a simulated middle-third fracture pattern were repaired with either an IM device (n = 10) or superiorly positioned nonlocking PS construct (n = 10). Loads were modeled to simulate physiologic load. Combined axial compression and torsion forces were sequentially increased until failure. Data were analyzed on the basis of loss of rotational stability using 3 criteria: early (10°), clinical (30°), and terminal (120°). Results: No significant difference was noted between constructs in early loss of rotational stability (P > .05). The PS group was significantly more rotationally stable than the IM group on the basis of clinical and terminal criteria (P < .05 for both). All test constructs failed in rotational stability. Conclusions: When tested under physiologic load, fixation failure occurred from loss of rotational stability. No statistical difference was seen between groups under early physiologic loads. However, during load to failure, the PS group was statistically more rotationally stable than the IM group. Given the clavicle’s function as a bony strut for the upper extremity and the biomechanical results demonstrated, rotational stability should be carefully considered during surgical planning and postoperative advancement of activity in patients undergoing operative fixation of middle-third clavicle fractures.
The views expressed are those of the authors and do not reflect the official policy of the Department of the Army, the Department of Defense, or the U.S. Government. This study was funded through an institutional grant from the Madigan Army Medical Center (MAMC), Department of Clinical Investigation (DCI). All tested implants were purchased using these funds through the MAMC DCI. This research was made possible through material and facility support from the Andersen Simulation Center at MAMC.
This study was approved by the Institutional Review Board of Madigan Army Medical Center, Department of Clinical Investigation (reference No. 211087). *Reprint requests: David J. Wilson, MD, Orthopaedic Surgery, Bassett Army Community Hospital, 1060 Gaffney Road #7400, Fort Wainwright, AK 99703-7400, USA. E-mail address: [email protected]; [email protected] (D.J. Wilson).
1058-2746/$ - see front matter Published by Elsevier Inc. on behalf of Journal of Shoulder and Elbow Surgery Board of Trustees. http://dx.doi.org/10.1016/j.jse.2015.10.006
ARTICLE IN PRESS 2
D.J. Wilson et al. Level of evidence: Basic Science Study; Biomechanics Published by Elsevier Inc. on behalf of Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Clavicle fractures; intramedullary fixation; biomechanical testing; shoulder; middle-third clavicle fractures; intramedullary implant stability
Clavicle fractures account for 2% to 4% of all adult fractures.8 Of these, approximately 80% occur within the middle third of the clavicle.7,17 Recent studies indicate that operative repair of selected clavicle fractures may improve rate of healing and prevent problems associated with malunions.2,9,11,17,21 When surgical treatment is indicated for a displaced midshaft clavicle fracture, a variety of implants may be selected, including plate-screw (PS) constructs and intramedullary (IM) devices. Clavicle plates can be further categorized by both the plate’s location-matching contour (superior, anteriorsuperior, anterior, and anterior-inferior) and the presence of locking or nonlocking screw options. Examples of the IM devices available for treatment of middle-third clavicle fractures include the 4.5-mm Herbert screw (Zimmer, Warsaw, IN, USA), the Rockwood (DePuy, Warsaw, IN, USA) or Hagie pin (2.5-, 3.0-, 3.8-, and 4.5-mm options; Smith & Nephew, Memphis, TN, USA), a 6.5-mm partially threaded cancellous screw, and the Sonoma CRx device (Sonoma Orthopedic Products Inc, Santa Rosa, CA, USA).1,12,16,22,23 Given the unique flat S shape of the clavicle, the complex deforming forces about the middle third of the bone, and its near-subcutaneous position, these various implant types have differing pros and cons. For example, superiorly positioned plates have been shown to be biomechanically superior to anterior and anterior-inferior plates, whereas the latter options offer safer drilling trajectories, affording the opportunity to place longer screws. Alternatively, IM implants are less prominent than plates and can be placed through smaller incisions with less soft tissue disruption. The Sonoma CRx nail is a relatively new implant that strives through unique design and features to maintain the advantages of IM clavicle fixation while improving the torsional resistance at the fracture site. The Wavibody design of the nail allows flexibility during implantation for ease of entry into the sinusoidal bone. On appropriate positioning, the nail can then be “actuated,” causing the implant to be become rigid once it is contoured to the clavicle. The actuating mechanism also triggers talons or grippers on the medial end of the implant to expand outward 8 mm to gain endosteal purchase. The lateral end of the implant is secured with a 2.7mm cortical screw. This process not only confers rotational stability on either side of the fracture but also generates compression at the fracture by the manner in which the grippers engage the surrounding bone. Previous studies have evaluated the biomechanical differences between these various fixation options. Most studies have used 3- and 4-point bending models to stress the hardware constructs, simulating a repeated injury mechanism.
Recently, 4 cases of early hardware failure were reported involving IM fixation using the Sonoma CRx device, raising concerns about the stability of this construct under low-demand conditions, as is generally experienced during early rehabilitation.13,23 Several authors have described this low-threshold physiologic stress and quantified it for the middle third of the clavicle. Taylor et al sought to qualify the normal physiologic forces across the midshaft of the clavicle during movements of the upper extremity required to feed oneself.19 The process of bringing the hand to the mouth was tested with a digitally simulated highly comminuted midshaft clavicle fracture (at a peak of 50° of internal rotation of the humerus, 15° of glenohumeral abduction, and 45° of forward flexion) using a computational model. The resulting computer analysis found that this complex movement produced axial compression and a rotational bending moment at the fracture site, resulting in downward and posterior displacement of the lateral fragment. Iannolo et al further quantified the magnitude of forces across the middle of the intact clavicle during shoulder motion using both computational and cadaveric biomechanical modeling.5 Their study found that during shoulder abduction, there was significantly more axial compression and torsion than with internal and external rotation of the humerus, producing pressures that reached, on average, a peak of 34 N of compression and 0.398 N·m of torque about the middle third of the clavicle.5 The objective of this study is to evaluate the ability of the Sonoma CRx nail to resist combined axial compression and torsion in a simulated midshaft clavicle fracture in comparison to a PS construct.
Methods The fourth-generation Sawbones (Pacific Research Laboratories, Vashon, WA, USA) model of the human clavicle was used for this biomechanical evaluation. This current model closely matches the modulus of elasticity and, as a result, the torsional and compressive stiffness of living human cortical and cancellous bone and has previously been successfully used to evaluate component stability of middle-third clavicle fracture models.14,15 A simulated interdigitating, transverse, middlethird fracture pattern was created in each of the 20 Sawbones models used. Equal groups of 10 clavicles were then fixated with either the Sonoma CRx IM device or a superiorly positioned nonlocking PS construct (3.5-mm reconstruction plate; DePuy Synthes, West Chester, PA, USA). All samples were then mounted in plaster of Paris and tested using a dynamic servohydraulic loading device (8521
ARTICLE IN PRESS Biomechanical analysis of midshaft clavicle fixation
3
Figure 2 Illustration depicting the clavicle sample mounted within the testing apparatus.
Figure 1 Illustration depicting the clavicle sample with a simulated interdigitating fracture pattern potted in plaster of Paris.
servohydraulic dynamic load device; Instron, Norwood, MA, USA). Once potted in matured plaster molds, the testing samples were oriented vertically along the longitudinal axis with the lateral portion affixed to the stable arm and the medial end to the actuating (mobile) arm of the Instron machine (Fig. 1). All compression force was applied along the longitudinal axis of the clavicle. All torsional force was applied in a clockwise direction around the longitudinal axis of the clavicle. Axial compressive force was measured along the x-axis by a linear displacement tensometer. Torsional force was measured along the y-axis by an angular displacement tensometer. Linear displacement along the x-axis and rotation displacement along the y-axis were measured and recorded by the Instron device itself through the actuating arm (Fig. 2). Physiologic stress was initiated with a combined 34 N of compression and 0.398 N·m of torque as described by Iannolo et al.5 These compression and torsion forces were applied in combination and sequentially increased until failure. A cyclic rate of 120/min was used. A slow ramp to peak load rate of 3 seconds was used at the beginning and end of each set. A
mean load rate of 50% was carried by each sample between cycles. Linear and rotational displacement data were collected for each cycle until failure. The first 300 cycles (set 1) was performed using our physiologic force parameters (combined 34 N compression and 0.398 N·m of torsion). After this set, the force applied to each sample surpassed the predetermined physiologic threshold and was increased in 300cycle increments until failure. Two primary data points were evaluated: first, the amount of rotational displacement (in degrees) created by physiologic force during the first 300 test cycles; and next, the total number of test cycles survived by each construct until terminal failure was observed. Terminal failure was defined as linear or rotational displacement that reached the limits of the testing device. The Instron machine used allowed a rotational arch of motion of 120. The number of cycles survived by each construct before 10° and before 30° of rotational displacement had occurred was also recorded. These prefailure data points were evaluated to compare to progressive loss of rotational stability leading to terminal failure.
Statistics There is a paucity of data regarding data variation available for fourth-generation Sawbones, but previous analyses using fourth-generation Sawbones have justified sample sizes of 5, 8, and 10.6,10,18,24 We determined that at a power of .8 and an α error of 5%, a sample size of 10 Sawbones was sufficient to determine a statistically significant difference between groups, assuming low variability. A P value < .05 was considered significant. The Student t test was used for statistical comparison, and a Z test was used to detect outliers. Stiffness is defined as a material’s resistance to deformation. In each of the constructs, the rotational stiffness (Nm/deg) was determined by calculating the slope of the force displacement curve in the elastic deformation range.
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Results All samples were tested to terminal failure. No divergence from the study protocol was recorded. The first 300 cycles were performed in a predetermined physiologic range. During this initial sequence, the average rotational displacement was 1.15° for the PS group vs. 1.14° for the IM group (P > .05). The average number of cycles before a 10° loss of rotational stability occurred was 1437 for the PS group vs. 1141 for the IM group (P > .05). The stiffness of the PS group was 0.84 Nm/deg vs. 0.51 Nm/deg (P > .05). The average number of cycles before a 30° loss of rotational stability was 1662 for the PS group vs. 1298 in the IM group (P < .05). The number of cycles before terminal failure was 1890 for the PS group vs. 1383 in the IM group (P < .05). The average torque at which the PS group failed was 5.27 N·m vs. 2.71 N·m in the IM group (P < .05). With all constructs, the terminal failure threshold for rotational stability (120° change from the start point) was met before any significant linear displacement was observed.
Discussion The preponderance of biomechanical literature previously published on middle-third clavicle fracture fixation has focused on 3-point and cantilever bending testing models.4,15,19 With this design, these studies primarily evaluated the ability of various implants to resist nonphysiologic forces, such as the stresses that would be generated from repeated traumatic injury. However, given that most standard postoperative protocols call for avoidance of high-demand and at-risk activities, such as contact sports and military combat training, until evidence of radiographic healing has been noted, these nonphysiologic high stresses are not commonly experienced in the postoperative period.8,9,17,20 Therefore, in response to the previously identified early postoperative failures of the Sonoma CR implant and using the information provided by prior studies concerning “normal physiologic stress” at the midshaft of the clavicle, this investigation was designed to test each fixation construct under low-stress, combined axial compression and torsion to mirror the normal demands experienced during early postoperative rehabilitation.5,13,19,23 Prior biomechanical comparisons between PS and IM implants for clavicle fixation have demonstrated a lack of torsional resistance inherent to the studied IM implants.3,4,15 Golish et al compared the Rockwood pin against a PS construct using a 4-point bending model and found significantly decreased stability with the IM device.4 Their study conclusion was that given these findings, the PS construct may be a more stable device with rehabilitation protocols focusing on repetitive movement in the early postoperative period.4 The Sonoma CRx implant has some novel features that are intended to help address this limitation. With the actu-
D.J. Wilson et al. ated semirigid device engaged along the length of the sinusoidal clavicle, points of fixation on either end of the implant, and additional stability conferred by the compression generated along a length-stable fracture pattern, the Sonoma CRx is a unique fixation device that provides fracture stability to bending, compression, and torsional stresses. The results of this investigation suggest that the Sonoma CR implant provides similar stability as a PS construct under physiologic loads, as defined previously. However, in load to failure testing, the PS construct was statistically more stable in rotation. This information can potentially be used to assist with implant selection and postoperative rehabilitation.
Weaknesses This study has several weaknesses. No matter the ingenuity and material quality used, biomechanical testing in a laboratory does not replicate human anatomy, physiology, and mechanics. Our clavicle models, setup, and testing model do not perfectly mirror a postoperative patient. However, the implants studied are a constant, and countless orthopedic implant biomechanical studies that predate this investigation have relied on similar simulated models to extrapolate data and to make clinical advances. Along similar lines, our chosen testing model represents just one of the countless motions and orientations of stressors that a postoperative clavicle construct would experience during the recovery period. We acknowledge this inherent limitation to our study and encourage further investigations of these implants in both the laboratory and clinical practice in the future. Another study limitation is that the sample size is small. The inherent variability in choosing a testing material is a key component to determining sample size. Because of the highly controlled process for synthesizing fourth-generation biomechanical Sawbones, this variability is significantly reduced. This fact has been cited in the statistical justification for sample sizes of <10 in several recent studies.6,10,18,24 In addition, this biomechanical setting may not represent the clinical scenario in which a delayed union results in an increased number of physiologic cycles exposed to the implant, causing fatigue failure. In the clinical setting in which there is additional concern for delayed union, an IM implant may be less advantageous than another implant that is more resistant to repeated physiologic loading over time.
Conclusion The results of this study demonstrated that under physiologic loading (ie, “normal physiologic” stress), there was no difference in rotational stability between the 2 constructs. In addition, the stiffness of the 2 constructs at physiologic loads was similar in the 2 groups. However, during load to failure, the PS group was statistically more stable in rotation than the IM group.
ARTICLE IN PRESS Biomechanical analysis of midshaft clavicle fixation Overall, the testing results involving physiologic loads, representing the demands typically experienced during the early postoperative period, are promising. Contrasting these results to prior published comparison studies, the Sonoma CRx implant performed superior in torsional stability to the previously studied IM implants compared with PS devices.3,4,15 Nonetheless, the load to failure data favored the PS construct over the Sonoma IM implant. Further investigation is required to determine the clinical significance of the elevated, potentially “supraphysiologic” loads used in the load to failure testing. Given the clavicle’s function as a bony strut for the upper extremity and the biomechanical results demonstrated, rotational stability should be carefully considered during surgical planning and postoperative advancement of activity in patients undergoing operative fixation of middle-third clavicle fractures.
Disclaimer The authors, their immediate families, and any research foundation with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.
References 1. Abo El Nor T. Displaced mid-shaft clavicular fractures: surgical treatment with intramedullary screw fixation. Arch Orthop Trauma Surg 2013;133:1395-9. http://dx.doi.org/10.1007/s00402-013-1809-3 2. Canadian Orthopedic Trauma Society. Nonoperative treatment compared with plate fixation of displaced midshaft clavicular fractures: a multicenter, randomized trial. J Bone Joint Surg Am 2007;89:1-10. http://dx.doi.org/10.2106/JBJS.F.00020 3. Drosdowech DS, Manwell SE, Ferreira LM, Goel DP, Faber KJ, Johnson JA. Biomechanical analysis of fixation of middle third fractures of the clavicle. J Orthop Trauma 2011;25:39-43. http://dx.doi.org/ 10.1097/BOT.0b013e3181d8893a 4. Golish R, Oliviero J, Francke E, Miller M. A biomechanical study of plate vs. intramedullary devices for midshaft clavicle fixation. J Orthop Surg Res 2008;3:28. http://dx.doi.org/10.1186/1749-799X-3-28 5. Iannolo M, Werner F, Sutton L, Serell S, VanValkenburg S. Forces across the middle of the intact clavicle during shoulder motion. J Shoulder Elbow Surg 2010;19:1013-7. http://dx.doi.org/10.1016/j.jse.2010.03.016 6. Kaiser MM, Wessel LM, Zachert G, Stratmann C, Eggert R, Gros N, et al. Biomechanica analysis of a synthetic femur spiral fracture model: influence of different materials on the stiffness in flexible intramedullary nailing. Clin Biomech 2011;26:592-7. http://dx.doi.org/10.1016/ j.clinbiomech.2011.01.012 7. Kemper A, Stitzel J, Gabler C, Duma S, Matsuoka F. Biomechanical response of the human clavicle subjected to dynamic bending. Biomed Sci Instrum 2006;42:231-6. 8. Khan LA, Bradnock TJ, Scott C, Robinson CM. Fractures of the clavicle. J Bone Joint Surg Am 2009;91:447-60. http://dx.doi.org/10.2106/ JBJS.H.00034
5 9. Kulshresthsa V, Roy T, Audige L. Operative versus nonoperative management of displaced midshaft clavicle fractures: a prospective cohort study. J Orthop Trauma 2011;25:31-8. http://dx.doi.org/10.1097/ BOT.0b013e3181d8290e 10. Lasanianos NG, Garnavos C, Magnisalis E, Kourkoulis S, Babis GC. A comparative biomechanical study for complex tibial plateau fractures: nailing and compression bolts versus modern and traditional plating. Injury 2013;44:133-9. http://dx.doi.org/10.1016/j.injusry.2013.03.013 11. Lenza M, Buchbinder R, Johnston RV, Belloti JC, Faloppa F. Surgical versus conservative interventions for treatment of fractures of the middle one third of the clavicle. Cochrane Database Syst Rev 2013;6:CD009363. http://dx.doi.org/10.1002/14651858.CD009363.pub2 12. Marlow WJ, Ralte P, Morapudi SP, Bassi R, Fischer J, Waseem M. Intramedullary fixation of diaphyseal clavicle fractures using the Rockwood clavicle pin: review of 86 cases. Open Orthop J 2012;6:482-7. http://dx.doi.org/10.2174/1874325001206010482 13. Palmer DK, Husain A, Phipatanakul WP, Wongworawat MD. Failure of a new intramedullary device in fixation of clavicle fractures: a report of two cases and review of the literature. J Shoulder Elbow Surg 2011;20:e1-4. http://dx.doi.org/10.1016/j.jse.2010.11.032 14. Partal G, Meyers KN, Sama N, Pagenkopf E, Lewis PB, Goldman A, et al. Superior versus anteroinferior plating of the clavicle revisited: a mechanical study. J Orthop Trauma 2010;24:420-5. http://dx.doi.org/ 10.1097/BOT.0b013e3181c3f6d4 15. Renfree T, Conrad B, Wright T. Biomechanical comparison of contemporary clavicle fixation devices. J Hand Surg Am 2010;35:639-44. http://dx.doi.org/10.1016/j.jhsa.2009.12.012 16. Richardson M, Asadollahi S, Richardson L. Management of acute displaced midshaft clavicular fractures using Herbert cannulated screw: technique and results in 114 patients. Int J Shoulder Surg 2013;7:52-8. http://dx.doi.org/10.4103/0973-6042.114227 17. Robinson CM, Goudie EB, Murray IR, Jenkins PJ, Ahktar MA, Read EO, et al. Open reduction and plate fixation versus nonoperative treatment for displaced midshaft clavicular fractures: a multicenter, randomized, controlled trial. J Bone Joint Surg Am 2013;95:1576-84. http://dx.doi.org/10.2106/JBJS.L.00307 18. Scolaro JA, Hsu JE, Svach DJ, Mehta S. Plate selection for fixation of extra-articular distal humerus fractures: a biomechanical comparison of three different implants. Injury 2014;45:2040-4. http://dx.doi.org/ 10.1016/j.injury.2014.08.036 19. Taylor P, Day R, Nicholls R, Rasmussen J, Yates P, Stoffel K. The comminuted midshaft clavicle fracture: a biomechanical evaluation of plating methods. Clin Biomech 2010;26:495-6. http://dx.doi.org/ 10.1016/j.clinbiomech.2010.12.007 20. Van der Meijden OA, Caskill TR, Millett PJ. Treatment of clavicle fractures: current concepts review. J Shoulder Elbow Surg 2012;21:423-9. http://dx.doi.org/10.1016/j.jse.2011.08.053 21. Virtanen KJ, Remes V, Pajarinen J, Savolainen V, Björkenheim JM, Paavola M. Sling compared with plate osteosynthesis for treatment of displaced midshaft clavicular fractures: a randomized clinical trial. J Bone Joint Surg Am 2012;94:1546-53. http://dx.doi.org/10.2106/JBJS.J.01999 22. Wenninger JJ Jr, Dannenbaum JH, Branstetter JG, Arrington ED. Comparison of complication rates of intramedullary pin fixation versus plating of midshaft clavicle fractures in an active duty military population. J Surg Orthop Adv 2013;22:77-81. http://dx.doi.org/10.3113/ jsoa.2013.0077 23. Wilson DJ, Weaver DL, Balog TP, Arrington ED. Early postoperative failure of a new intramedullary fixation device for midshaft clavicle fractures. Orthopedics 2013;36:e1450-3. http://dx.doi.org/10.3928/ 01477447-20131021-31 24. Zachert G, Rapp M, Eggert R, Schulze-Hessing M, Gros N, Stratmann C, et al. Additional tension screws improve stability in elastic stable intramedullary nailing: biomechanical analysis of a femur spiral fracture model. Eur J Pediatr Surg 2015;25:365-72. http://dx.doi.org/10.1055/s0034-1376394
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Biomechanical analysis of intramedullary vs. superior plate fixation of transverse midshaft clavicle fractures David J. Wilson, MDa,*, William F. Scully, MDb, Kyong S. Min, MDc, Tess A. Harmon, BAd, Josef K. Eichinger, MDa, Edward D. Arrington, MDa a
Madigan Army Medical Center, Joint Base Lewis McChord, WA, USA Martin Army Community Hospital, Ft. Benning, GA, USA c Brian Allgood Army Community Hospital, Yongsan, South Korea d Loma Linda University School of Medicine, Loma Linda, CA, USA b
Background: Middle-third clavicle fractures represent 2% to 4% of all skeletal trauma in the United States. Treatment options include intramedullary (IM) as well as plate and screw (PS) constructs. The purpose of this study was to analyze the biomechanical stability of a specific IM system compared with nonlocking PS fixation under low-threshold physiologic load. Methods: Twenty fourth-generation Sawbones (Pacific Research Laboratories, Vashon, WA, USA) with a simulated middle-third fracture pattern were repaired with either an IM device (n = 10) or superiorly positioned nonlocking PS construct (n = 10). Loads were modeled to simulate physiologic load. Combined axial compression and torsion forces were sequentially increased until failure. Data were analyzed on the basis of loss of rotational stability using 3 criteria: early (10°), clinical (30°), and terminal (120°). Results: No significant difference was noted between constructs in early loss of rotational stability (P > .05). The PS group was significantly more rotationally stable than the IM group on the basis of clinical and terminal criteria (P < .05 for both). All test constructs failed in rotational stability. Conclusions: When tested under physiologic load, fixation failure occurred from loss of rotational stability. No statistical difference was seen between groups under early physiologic loads. However, during load to failure, the PS group was statistically more rotationally stable than the IM group. Given the clavicle’s function as a bony strut for the upper extremity and the biomechanical results demonstrated, rotational stability should be carefully considered during surgical planning and postoperative advancement of activity in patients undergoing operative fixation of middle-third clavicle fractures.
The views expressed are those of the authors and do not reflect the official policy of the Department of the Army, the Department of Defense, or the U.S. Government. This study was funded through an institutional grant from the Madigan Army Medical Center (MAMC), Department of Clinical Investigation (DCI). All tested implants were purchased using these funds through the MAMC DCI. This research was made possible through material and facility support from the Andersen Simulation Center at MAMC.
This study was approved by the Institutional Review Board of Madigan Army Medical Center, Department of Clinical Investigation (reference No. 211087). *Reprint requests: David J. Wilson, MD, Orthopaedic Surgery, Bassett Army Community Hospital, 1060 Gaffney Road #7400, Fort Wainwright, AK 99703-7400, USA. E-mail address: [email protected]; [email protected] (D.J. Wilson).
1058-2746/$ - see front matter Published by Elsevier Inc. on behalf of Journal of Shoulder and Elbow Surgery Board of Trustees. http://dx.doi.org/10.1016/j.jse.2015.10.006
ARTICLE IN PRESS 2
D.J. Wilson et al. Level of evidence: Basic Science Study; Biomechanics Published by Elsevier Inc. on behalf of Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Clavicle fractures; intramedullary fixation; biomechanical testing; shoulder; middle-third clavicle fractures; intramedullary implant stability
Clavicle fractures account for 2% to 4% of all adult fractures.8 Of these, approximately 80% occur within the middle third of the clavicle.7,17 Recent studies indicate that operative repair of selected clavicle fractures may improve rate of healing and prevent problems associated with malunions.2,9,11,17,21 When surgical treatment is indicated for a displaced midshaft clavicle fracture, a variety of implants may be selected, including plate-screw (PS) constructs and intramedullary (IM) devices. Clavicle plates can be further categorized by both the plate’s location-matching contour (superior, anteriorsuperior, anterior, and anterior-inferior) and the presence of locking or nonlocking screw options. Examples of the IM devices available for treatment of middle-third clavicle fractures include the 4.5-mm Herbert screw (Zimmer, Warsaw, IN, USA), the Rockwood (DePuy, Warsaw, IN, USA) or Hagie pin (2.5-, 3.0-, 3.8-, and 4.5-mm options; Smith & Nephew, Memphis, TN, USA), a 6.5-mm partially threaded cancellous screw, and the Sonoma CRx device (Sonoma Orthopedic Products Inc, Santa Rosa, CA, USA).1,12,16,22,23 Given the unique flat S shape of the clavicle, the complex deforming forces about the middle third of the bone, and its near-subcutaneous position, these various implant types have differing pros and cons. For example, superiorly positioned plates have been shown to be biomechanically superior to anterior and anterior-inferior plates, whereas the latter options offer safer drilling trajectories, affording the opportunity to place longer screws. Alternatively, IM implants are less prominent than plates and can be placed through smaller incisions with less soft tissue disruption. The Sonoma CRx nail is a relatively new implant that strives through unique design and features to maintain the advantages of IM clavicle fixation while improving the torsional resistance at the fracture site. The Wavibody design of the nail allows flexibility during implantation for ease of entry into the sinusoidal bone. On appropriate positioning, the nail can then be “actuated,” causing the implant to be become rigid once it is contoured to the clavicle. The actuating mechanism also triggers talons or grippers on the medial end of the implant to expand outward 8 mm to gain endosteal purchase. The lateral end of the implant is secured with a 2.7mm cortical screw. This process not only confers rotational stability on either side of the fracture but also generates compression at the fracture by the manner in which the grippers engage the surrounding bone. Previous studies have evaluated the biomechanical differences between these various fixation options. Most studies have used 3- and 4-point bending models to stress the hardware constructs, simulating a repeated injury mechanism.
Recently, 4 cases of early hardware failure were reported involving IM fixation using the Sonoma CRx device, raising concerns about the stability of this construct under low-demand conditions, as is generally experienced during early rehabilitation.13,23 Several authors have described this low-threshold physiologic stress and quantified it for the middle third of the clavicle. Taylor et al sought to qualify the normal physiologic forces across the midshaft of the clavicle during movements of the upper extremity required to feed oneself.19 The process of bringing the hand to the mouth was tested with a digitally simulated highly comminuted midshaft clavicle fracture (at a peak of 50° of internal rotation of the humerus, 15° of glenohumeral abduction, and 45° of forward flexion) using a computational model. The resulting computer analysis found that this complex movement produced axial compression and a rotational bending moment at the fracture site, resulting in downward and posterior displacement of the lateral fragment. Iannolo et al further quantified the magnitude of forces across the middle of the intact clavicle during shoulder motion using both computational and cadaveric biomechanical modeling.5 Their study found that during shoulder abduction, there was significantly more axial compression and torsion than with internal and external rotation of the humerus, producing pressures that reached, on average, a peak of 34 N of compression and 0.398 N·m of torque about the middle third of the clavicle.5 The objective of this study is to evaluate the ability of the Sonoma CRx nail to resist combined axial compression and torsion in a simulated midshaft clavicle fracture in comparison to a PS construct.
Methods The fourth-generation Sawbones (Pacific Research Laboratories, Vashon, WA, USA) model of the human clavicle was used for this biomechanical evaluation. This current model closely matches the modulus of elasticity and, as a result, the torsional and compressive stiffness of living human cortical and cancellous bone and has previously been successfully used to evaluate component stability of middle-third clavicle fracture models.14,15 A simulated interdigitating, transverse, middlethird fracture pattern was created in each of the 20 Sawbones models used. Equal groups of 10 clavicles were then fixated with either the Sonoma CRx IM device or a superiorly positioned nonlocking PS construct (3.5-mm reconstruction plate; DePuy Synthes, West Chester, PA, USA). All samples were then mounted in plaster of Paris and tested using a dynamic servohydraulic loading device (8521
ARTICLE IN PRESS Biomechanical analysis of midshaft clavicle fixation
3
Figure 2 Illustration depicting the clavicle sample mounted within the testing apparatus.
Figure 1 Illustration depicting the clavicle sample with a simulated interdigitating fracture pattern potted in plaster of Paris.
servohydraulic dynamic load device; Instron, Norwood, MA, USA). Once potted in matured plaster molds, the testing samples were oriented vertically along the longitudinal axis with the lateral portion affixed to the stable arm and the medial end to the actuating (mobile) arm of the Instron machine (Fig. 1). All compression force was applied along the longitudinal axis of the clavicle. All torsional force was applied in a clockwise direction around the longitudinal axis of the clavicle. Axial compressive force was measured along the x-axis by a linear displacement tensometer. Torsional force was measured along the y-axis by an angular displacement tensometer. Linear displacement along the x-axis and rotation displacement along the y-axis were measured and recorded by the Instron device itself through the actuating arm (Fig. 2). Physiologic stress was initiated with a combined 34 N of compression and 0.398 N·m of torque as described by Iannolo et al.5 These compression and torsion forces were applied in combination and sequentially increased until failure. A cyclic rate of 120/min was used. A slow ramp to peak load rate of 3 seconds was used at the beginning and end of each set. A
mean load rate of 50% was carried by each sample between cycles. Linear and rotational displacement data were collected for each cycle until failure. The first 300 cycles (set 1) was performed using our physiologic force parameters (combined 34 N compression and 0.398 N·m of torsion). After this set, the force applied to each sample surpassed the predetermined physiologic threshold and was increased in 300cycle increments until failure. Two primary data points were evaluated: first, the amount of rotational displacement (in degrees) created by physiologic force during the first 300 test cycles; and next, the total number of test cycles survived by each construct until terminal failure was observed. Terminal failure was defined as linear or rotational displacement that reached the limits of the testing device. The Instron machine used allowed a rotational arch of motion of 120. The number of cycles survived by each construct before 10° and before 30° of rotational displacement had occurred was also recorded. These prefailure data points were evaluated to compare to progressive loss of rotational stability leading to terminal failure.
Statistics There is a paucity of data regarding data variation available for fourth-generation Sawbones, but previous analyses using fourth-generation Sawbones have justified sample sizes of 5, 8, and 10.6,10,18,24 We determined that at a power of .8 and an α error of 5%, a sample size of 10 Sawbones was sufficient to determine a statistically significant difference between groups, assuming low variability. A P value < .05 was considered significant. The Student t test was used for statistical comparison, and a Z test was used to detect outliers. Stiffness is defined as a material’s resistance to deformation. In each of the constructs, the rotational stiffness (Nm/deg) was determined by calculating the slope of the force displacement curve in the elastic deformation range.
ARTICLE IN PRESS 4
Results All samples were tested to terminal failure. No divergence from the study protocol was recorded. The first 300 cycles were performed in a predetermined physiologic range. During this initial sequence, the average rotational displacement was 1.15° for the PS group vs. 1.14° for the IM group (P > .05). The average number of cycles before a 10° loss of rotational stability occurred was 1437 for the PS group vs. 1141 for the IM group (P > .05). The stiffness of the PS group was 0.84 Nm/deg vs. 0.51 Nm/deg (P > .05). The average number of cycles before a 30° loss of rotational stability was 1662 for the PS group vs. 1298 in the IM group (P < .05). The number of cycles before terminal failure was 1890 for the PS group vs. 1383 in the IM group (P < .05). The average torque at which the PS group failed was 5.27 N·m vs. 2.71 N·m in the IM group (P < .05). With all constructs, the terminal failure threshold for rotational stability (120° change from the start point) was met before any significant linear displacement was observed.
Discussion The preponderance of biomechanical literature previously published on middle-third clavicle fracture fixation has focused on 3-point and cantilever bending testing models.4,15,19 With this design, these studies primarily evaluated the ability of various implants to resist nonphysiologic forces, such as the stresses that would be generated from repeated traumatic injury. However, given that most standard postoperative protocols call for avoidance of high-demand and at-risk activities, such as contact sports and military combat training, until evidence of radiographic healing has been noted, these nonphysiologic high stresses are not commonly experienced in the postoperative period.8,9,17,20 Therefore, in response to the previously identified early postoperative failures of the Sonoma CR implant and using the information provided by prior studies concerning “normal physiologic stress” at the midshaft of the clavicle, this investigation was designed to test each fixation construct under low-stress, combined axial compression and torsion to mirror the normal demands experienced during early postoperative rehabilitation.5,13,19,23 Prior biomechanical comparisons between PS and IM implants for clavicle fixation have demonstrated a lack of torsional resistance inherent to the studied IM implants.3,4,15 Golish et al compared the Rockwood pin against a PS construct using a 4-point bending model and found significantly decreased stability with the IM device.4 Their study conclusion was that given these findings, the PS construct may be a more stable device with rehabilitation protocols focusing on repetitive movement in the early postoperative period.4 The Sonoma CRx implant has some novel features that are intended to help address this limitation. With the actu-
D.J. Wilson et al. ated semirigid device engaged along the length of the sinusoidal clavicle, points of fixation on either end of the implant, and additional stability conferred by the compression generated along a length-stable fracture pattern, the Sonoma CRx is a unique fixation device that provides fracture stability to bending, compression, and torsional stresses. The results of this investigation suggest that the Sonoma CR implant provides similar stability as a PS construct under physiologic loads, as defined previously. However, in load to failure testing, the PS construct was statistically more stable in rotation. This information can potentially be used to assist with implant selection and postoperative rehabilitation.
Weaknesses This study has several weaknesses. No matter the ingenuity and material quality used, biomechanical testing in a laboratory does not replicate human anatomy, physiology, and mechanics. Our clavicle models, setup, and testing model do not perfectly mirror a postoperative patient. However, the implants studied are a constant, and countless orthopedic implant biomechanical studies that predate this investigation have relied on similar simulated models to extrapolate data and to make clinical advances. Along similar lines, our chosen testing model represents just one of the countless motions and orientations of stressors that a postoperative clavicle construct would experience during the recovery period. We acknowledge this inherent limitation to our study and encourage further investigations of these implants in both the laboratory and clinical practice in the future. Another study limitation is that the sample size is small. The inherent variability in choosing a testing material is a key component to determining sample size. Because of the highly controlled process for synthesizing fourth-generation biomechanical Sawbones, this variability is significantly reduced. This fact has been cited in the statistical justification for sample sizes of <10 in several recent studies.6,10,18,24 In addition, this biomechanical setting may not represent the clinical scenario in which a delayed union results in an increased number of physiologic cycles exposed to the implant, causing fatigue failure. In the clinical setting in which there is additional concern for delayed union, an IM implant may be less advantageous than another implant that is more resistant to repeated physiologic loading over time.
Conclusion The results of this study demonstrated that under physiologic loading (ie, “normal physiologic” stress), there was no difference in rotational stability between the 2 constructs. In addition, the stiffness of the 2 constructs at physiologic loads was similar in the 2 groups. However, during load to failure, the PS group was statistically more stable in rotation than the IM group.
ARTICLE IN PRESS Biomechanical analysis of midshaft clavicle fixation Overall, the testing results involving physiologic loads, representing the demands typically experienced during the early postoperative period, are promising. Contrasting these results to prior published comparison studies, the Sonoma CRx implant performed superior in torsional stability to the previously studied IM implants compared with PS devices.3,4,15 Nonetheless, the load to failure data favored the PS construct over the Sonoma IM implant. Further investigation is required to determine the clinical significance of the elevated, potentially “supraphysiologic” loads used in the load to failure testing. Given the clavicle’s function as a bony strut for the upper extremity and the biomechanical results demonstrated, rotational stability should be carefully considered during surgical planning and postoperative advancement of activity in patients undergoing operative fixation of middle-third clavicle fractures.
Disclaimer The authors, their immediate families, and any research foundation with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.
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