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Optimal Fixation of Oblique Scaphoid Fractures: A Cadaver Model
SCIENTIFIC ARTICLE
Optimal Fixation of Oblique Scaphoid Fractures: A Cadaver Model Shai Luria, MD, Lado Lenart, PhD, Borut Lenart, MME, Eran Peleg, PhD, Matej Kastelec, MD
Purpose Acute scaphoid fractures are commonly fixed with headless cannulated screws positioned in the center of the proximal fragment. Central placement of the screw may be difficult and may violate the scaphotrapezial joint. We hypothesize that placement of the screw through the scaphoid tuberosity will achieve perpendicular fixation of an oblique waist fracture and result in more stable fixation than a screw in the center of the proximal fragment. Methods We designed oblique osteotomies for 8 matched pairs of cadaver scaphoids and fixed each specimen with a headless cannulated screw. In 1 specimen, we positioned the screw at the center of the proximal fragment; we placed its matched pair perpendicular to the fracture. The perpendicular screw was directed through the scaphoid tuberosity. We placed the specimen under the increasing load of a pneumatically driven plunger. We compared stiffness, load, distance at failure, and mechanism of failure between the central and perpendicular screw groups. Results We found no difference between groups. Stiffness was identical in both groups (131 N/mm) and load to failure was similar (central screw, 137 N vs perpendicular screw, 148 N). Conclusions In this biomechanical model of an unstable scaphoid fracture, we found that similar stability of fixation had been achieved with a screw perpendicular to the fracture plane with entry through the tuberosity, compared with a screw in a central position in the proximal fragment. This study suggests that placing the screw through the tuberosity, perpendicular to a short oblique fracture, will not impair fixation stability. Clinical relevance Percutaneous fixation of scaphoid fractures has become popular although it is technically challenging. An easier distal approach through the tuberosity, without violating the scaphotrapezial joint, may not impair the fixation stability of an oblique fracture. (J Hand Surg 2012;37A:1400–1404. Copyright © 2012 by the American Society for Surgery of the Hand. All rights reserved.) Key words Biomechanics, internal fixation, scaphoid fracture, wrist. HE OPTIMAL MODE of fixation of scaphoid fractures continues to be debated.1– 8 Percutaneous fixation of scaphoid fractures has become popular for treating undisplaced or minimally displaced fractures.9 More recently, others have used this technique for displaced fractures and nonunions.7
T
From the Departments of Orthopaedic Surgery and Medical Engineering, Hadassah–Hebrew University Medical Center, Kiryat Hadassah, Jerusalem, Israel; and Institute “Jožef Štefan” and the Department of Traumatology, University Clinical Center Ljubljana, Ljubljana, Slovenia. The authors acknowledge Synthes GmbH, Switzerland, for funding travel expenses, screws, and cadavers for this study. Received for publication December 5, 2011; accepted in revised form April 15, 2012.
1400 䉬 © ASSH 䉬 Published by Elsevier, Inc. All rights reserved.
Central placement of a headless cannulated screw in the proximal fragment of the scaphoid has become common practice and is considered superior biomechanically and clinically.1,10,11 Proper positioning of the screw in this position may be particularly demanding in a distal to proximal percutaneous technique.3,6 In No benefits in any form have been received or will be received related directly or indirectly to the subject of this article. Corresponding author: Shai Luria, MD, Department of Orthopaedic Surgery, Hadassah–Hebrew University Medical Center, Kiryat Hadassah, POB 12000, Jerusalem 91120, Israel; e-mail: [email protected]. 0363-5023/12/37A07-0015$36.00/0 http://dx.doi.org/10.1016/j.jhsa.2012.04.021
OPTIMAL FIXATION OF OBLIQUE SCAPHOID FRACTURE
an open distal approach, adequate positioning is possible through the scaphotrapezial joint. This positioning may be achieved with resection of the base of the trapezium or through the trapezium itself.12 Central placement of the screw in the proximal fragment has been evaluated on a cadaver model of transverse waist fractures without addressing other fracture configurations.1 Previously, a finite element analysis model showed that the most stable fixation may be achieved by placing a screw perpendicular to the fracture plane and not necessarily parallel to the long axis of the scaphoid. This difference was especially pronounced in an oblique fracture.5 The aim of this cadaver study was to compare fixation of an oblique scaphoid fracture using a screw perpendicular to the fracture, entering through the scaphoid tuberosity with a screw centrally located in the proximal fragment. Specifically, we aimed to determine whether a percutaneous screw placed through the tuberosity and perpendicular to a fracture could achieve adequate stability. Our global null hypothesis was that there would be no difference in stability between the 2 types of fixation. MATERIALS AND METHODS We removed 8 matched pairs of scaphoids from fresh cadaver wrists for comparison. Each pair consisted of a right and a left scaphoid from 1 individual. We performed an oblique osteotomy on each scaphoid and fixed it with a screw either perpendicular to the osteotomy and through the tuberosity or in the center of the proximal fragment. We applied load to the specimens to evaluate the stability of the fixation. The outcome variables were load at failure of fixation and mechanism of failure. Unstable fracture simulating osteotomy We cleaned the scaphoids of all soft tissue and determined the density of each scaphoid according to Archimedes’ principle (stating that the volume of displaced fluid is the volume of the object, measured as mass per volume). We randomly assigned the scaphoids within each pair to placement either central in the proximal fragment or perpendicular to the screw. For each matched pair, we fixed the complementary scaphoid with the opposite screw position, either central or perpendicular. We marked each specimen circumferentially about the narrowest aspect of the scaphoid waist, transverse to the general axis of the bone, using a 3-0 nylon thread as a guide. We marked the volar side of each scaphoid (marked by the tuberosity) 2 mm proximal to the midline mark, marked the dorsal (opposite)
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surface 2 mm distal to that line. We made a line between these 2 points on either side of the bone. We marked the osteotomy site on each specimen and planned the desired form of screw placement before we performed the osteotomy. Each scaphoid was placed in a vice, and a smooth oblique cut with a thin-blade, pneumatically driven circular bone saw created an oblique fracture.13 Two types of fixation of the simulated fracture After the osteotomy, we drilled a single Kirschner wire through each specimen according to the study group. We made high-resolution radiographs in the anteroposterior (AP) and lateral planes to confirm correct placement of the wire (Fig. 1). To standardize the radiographs taken, the tuberosity marked the volar aspect of the bone and the articulation surface with the capitate, the medial aspect. We defined central positional placement of the wire for the central screw as a wire position within the central one-third of the proximal fragment on both the AP and lateral radiographs.1 On each radiograph, we divided the section of the wire through the proximal fragment into thirds and evaluated the location of the wire in the central third of the bone at 2 equal points along this part of the wire (one-third height and twothirds height) (Fig. 1). At each of these points, the distance between the wire and cortices of the bone was calculated and expressed as a fraction of the entire width of bone at that location. This was expressed on a scale of 0 to 1, where 0 or 1 was a wire adjacent to the side of the bone and 0.5 was exactly central at this location. Using this method, the central third ranged between approximately 0.33 and 0.66. We defined radiographic evaluation of the wire for the perpendicular screw as placement that was perpendicular to the fracture and within the central one-third of the osteotomy plane on the AP and lateral views (Fig. 1). We measured this according to the same principle as described above, and expressed it as a fraction of the distance of the wire (crossing the fracture) to the cortices and the entire width of bone along the osteotomy line. After we placed the wire satisfactorily, we inserted the appropriately sized headless cannulated screw over the wire, according to the manufacturer’s guide (HCS 2.4; Synthes GmbH, Oberdorf, Switzerland). We inserted all screws to 2-finger tightness and made additional radiographs in the AP and lateral planes to confirm placement, and to verify that all proximal screw threads crossed the osteotomy.
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FIGURE 1: After cutting the oblique osteotomy, we placed a Kirschner wire to guide the cannulated screw. The left images are lateral and anteroposterior radiographs of the scaphoid with a perpendicular fixation; the right images are of the fixation in the center of the proximal fragment. Note the direction of the perpendicular wire toward the scaphoid tuberosity.
Testing of fixation stability For mechanical testing, we potted the proximal fragment of each specimen in a holder with polyurethane (Fig. 2). We passed a Kirschner wire through the proximal end of the scaphoid to provide additional anchoring stability before potting the specimen.1 The scaphoid was oriented at a 45° angle to the horizontal plane of the holder, to mimic its normal attitude in a wrist held in the neutral position. This enabled the delivery of a dorsalto-volar cantilever load, which represents the primary physiologic load encountered by the scaphoid.1,14 We then placed each specimen into a fixture placed on a load-cell (9281CA; Kistler Instrumente AG, Winterthur, Switzerland). A pneumatically driven plunger resting on the surface of the distal pole produced a load with a constantly increasing excursion at a rate of 0.014 mm/s. We measured the excursion of the plunger (DNCE; Festo, Esslingen, Germany) by a linear variable motor controller (CMMP-AS-C23A; Festo). We increased the load until the fixation failed by fracture and loss of reduction. We recorded load at failure and mechanisms of failure on a personal computer with data acquisition hardware and software (Visual C⫹⫹, Visual Studio; Microsoft,
Redmond, WA), via a Gateway sysWORKS (type CAN; Norton-Symantec, Mountain View, CA). We calculated stiffness from the load and displacement measurements. Estimation of sample size and power We calculated the sample sizes assuming equal sizes of both study samples, with ␣ ⫽ 0.05,  ⫽ 0.20 (for 80% power with 95% confidence), and D ⫽ 0.40 (for standard deviation). The sample size computed resulted in 8 pairs of scaphoids. This sample size was consistent with a previous power analysis performed using biomechanical testing data, which showed that 8 to 12 runs with each screw should provide significant data.1,15,16 Data analysis We used the Wilcoxon signed-rank nonparametric analysis to compare the stiffness and strength of the specimens. Statistical significance was established at P ⬎ .05. RESULTS We found no difference between the mean density of the left and right scaphoid pairs. The mean density was
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at the 2 points measured; in the lateral views, the mean was 0.49 (SD, 0.10) and 0.5 (SD, 0.75), with a range of 0.36 to 0.63 at the 2 points measured. Biomechanical testing demonstrated no significant difference between placements of the screw in the center of the proximal fragment of the scaphoid compared with a screw positioned perpendicular to the fracture plane. Load to failure of the fixation was 137 ⫾ 69 N for the central screw, compared with 148 ⫾ 49 N for the perpendicular screw. Displacement of fracture at failure was 1.2 ⫾ 0.5 mm for the central screw and 1.5 ⫾ 0.3 mm for the perpendicular screw, which was an insignificant difference. Stiffness was similar between groups (131 ⫾ 59 N/mm for the central screw vs 131 ⫾ 41 N/mm for the perpendicular screw). The mode of failure for all specimens was screw migration and fracture at the screw– bone interface, with the distal fragment sliding along the oblique fracture plane. None of the specimens dislodged from the potting fixture. Upon retrieval of the screws after failure, none were bent.
FIGURE 2: Schematic depiction of the potted scaphoid specimen and testing apparatus for delivery of a dorsal-tovolar cantilever bending load.
0.47 g/cm3 (SD, ⫾ 0.07) for the perpendicular fixed specimens and 0.47 g/cm3 (SD, ⫾ 0.05) for the centrally fixed specimens. Therefore, it was not necessary to normalize measurements of load and stiffness to bone density. We found the screw sizes used to be similar: 21.5 ⫾ 1.8 mm for the central screw and 21.5 ⫾ 2.6 mm for the perpendicular screw. Radiographic evaluation of all of the specimens adhered to the criteria described above for the placement of the screws (Fig. 1). The average angle between the fracture and wire was 4° (SD, 3°) on the AP view and 4° (SD, 3°) on the lateral view for the perpendicular fixation, and 19° (SD, 16°) on the AP view and 11° (SD, 11°) on the lateral view for the central fixation. In the perpendicular fixation group, all wires were directed through the central third of the fracture (mean, 0.5; SD, 0.1). In the central screw group, all wires passed through the central third of the proximal fragment of the specimen. The calculated location of the wires in the central third of the proximal fragment were as follows (0.5 is a perfect central location at the point measured): In the anteroposterior views, the mean was 0.46 (SD, 0.04) and 0.51 (SD, 0.05), with a range of 0.40 to 0.57
DISCUSSION The purpose of this study was to evaluate the biomechanical properties of a different approach to the fixation of acute scaphoid fractures. Our hypotheses were that a distal approach through the tuberosity and not through the scaphotrapezial joint will not impair fixation stability, and that a screw placed perpendicular to the fracture plane will result in improved stability compared with a screw placed in the center of the proximal fragment, as previously suggested.5,6 According to this study, fixation of a scaphoid fracture through the tuberosity and perpendicular to the fracture plane resulted in comparable stability to a screw placed in the center of the proximal fragment. Not only can this simplify the percutaneous distal fixation technique, it may also make it unnecessary to disturb the scaphotrapezial joint or resection of part of the trapezial base to achieve desirable screw positioning. Although we found that higher loads were necessary to displace the fracture with the perpendicular screw compared with a screw in the center of the proximal fragment, this was not significant statistically. A finite element model compared the biomechanical properties of the fixation of acute scaphoid fractures of different configurations with a screw placed perpendicular versus central in the proximal fracture fragment.5 This study reported that the perpendicular screw had a biomechanical advantage, and that this increased with the more oblique fractures. For this reason, we designed the current cadaver study with a short oblique unstable
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configuration, to accentuate the difference in location of the perpendicular and central screws. The design of this grossly unstable fracture resulted in lower loads to failure compared with previous studies. The loads measured in this study averaged 137 N for the central screw and 148 N for the perpendicular screw. McCallister et al1 evaluated more stable transverse fractures that resulted in loads between 500 and 700 N. Fulkerson et al17 resected a volar wedge in the scaphoid to simulate the humpback deformity of a fracture nonunion. This resulted in lower loads to displacement (approximately 200 – 400 N, measured at 4-mm displacement). A wedge of this shape is probably less stable than the simple transverse osteotomy. This cadaver study did not show an advantage to a perpendicular screw compared with the finite element model. There may be several reasons for the difference between the studies. The computerized model is superior in its precision compared with the cadaver model. It is possible that the insignificant difference in favor of the perpendicular fixation found in the cadaver study would be significant in a larger study group. In contrast, the finite element method is only as good as the factors taken into account in its design. While performing the cadaver study, we noticed that the central screw may be directed more parallel to the plunger used as the displacing force. In this situation, the screw itself may have contributed more resistance when it was placed in a more parallel direction, although we cannot prove this hypothesis. There are several limitations to this study, as with any cadaver study. We isolated the specimens of all surrounding tissue and evaluated only stability of fixation. We simplified the forces mimicking physiological forces into a single vector and did not perform cyclic loading. In addition, the linear osteotomy chosen does not mimic the comminution or interdigitation possible in an actual fracture, which affects its stability. According to the results of this cadaver model, a screw placed through the scaphoid tuberosity and perpendicular to the fracture will result in stability similar to that of a screw placed in the center of the proximal scaphoid that is aimed at the scaphotrapezial joint.
REFERENCES 1. McCallister WV, Knight J, Kaliappan R, Trumble TE. Central placement of the screw in simulated fractures of the scaphoid waist: a biomechanical study. J Bone Joint Surg 2003;85A:72–77. 2. Ezquerro F, Jimenez S, Perez A, Prado M, de Diego G, Simon A. The influence of wire positioning upon the initial stability of scaphoid fractures fixed using Kirschner wires. A finite element study. Med Eng Phys 2007;29:652– 660. 3. Jeon I-H, Micic ID, Oh C-W, Park B-C, Kim P-T. Percutaneous screw fixation for scaphoid fracture: a comparison between the dorsal and the volar approaches. J Hand Surg 2009;34A:228 –236. 4. Leventhal EL, Wolfe SW, Walsh EF, Crisco JJ. A computational approach to the “optimal” screw axis location and orientation in the scaphoid bone. J Hand Surg 2009;34A:677– 684. 5. Luria S, Hoch S, Liebergall M, Mosheiff R, Peleg E. Optimal fixation of acute scaphoid fractures: finite element analysis. J Hand Surg 2010;35A:1246 –1250. 6. Soubeyrand M, Biau D, Mansour C, Mahjoub S, Molina V, Gagey O. Comparison of percutaneous dorsal versus volar fixation of scaphoid waist fractures using a computer model in cadavers. J Hand Surg 2009;34A:1838 –1844. 7. Merrell G, Slade J. Technique for percutaneous fixation of displaced and nondisplaced acute scaphoid fractures and select nonunions. J Hand Surg 2008;33A:966 –973. 8. Slade Iii JF, Grauer JN, Mahoney JD. Arthroscopic reduction and percutaneous fixation of scaphoid fractures with a novel dorsal technique. Orthop Clin North Am 2001;32:247–261. 9. Chan KW, McAdams TR. Central screw placement in percutaneous screw scaphoid fixation: a cadaveric comparison of proximal and distal techniques. J Hand Surg 2004;29A:74 –79. 10. Trumble TE, Gilbert M, Murray LW, Smith J, Rafijah G, McCallister WV. Displaced scaphoid fractures treated with open reduction and internal fixation with a cannulated screw. J Bone Joint Surg 2000; 82A:633– 641. 11. Chang MA, Bishop AT, Moran SL, Shin AY. The outcomes and complications of 1,2-intercompartmental supraretinacular artery pedicled vascularized bone grafting of scaphoid nonunions. J Hand Surg 2006;31A:387–396. 12. Geurts G, van Riet R, Meermans G, Verstreken F. Incidence of scaphotrapezial arthritis following volar percutaneous fixation of nondisplaced scaphoid waist fractures using a transtrapezial approach. J Hand Surg 2011;36A:1753–1758. 13. Szabo RMMD, Manske DMD. Displaced fractures of the scaphoid. Clin Orthop Relat Res 1988;230:30 –38. 14. Green DP. Green’s operative hand surgery. 5th ed. Philadelphia: Elsevier/Churchill Livingstone, 2005. 15. Shaw JA. A biomechanical comparison of scaphoid screws. J Hand Surg 1987;12A:347–353. 16. Newport ML, Williams CD, Bradley WD. Mechanical strength of scaphoid fixation. J Hand Surg 1996;21B:99 –102. 17. Fulkerson E, Keshner M, Kubiak EN, Paksima N, Panchal A. Comparison of fixation methods for scaphoid nonunions: a biomechanical model. Bull NYU Hosp Jt Dis 2007;65:271–275.
JHS 䉬 Vol A, July
Optimal Fixation of Oblique Scaphoid Fractures: A Cadaver Model Shai Luria, MD, Lado Lenart, PhD, Borut Lenart, MME, Eran Peleg, PhD, Matej Kastelec, MD
Purpose Acute scaphoid fractures are commonly fixed with headless cannulated screws positioned in the center of the proximal fragment. Central placement of the screw may be difficult and may violate the scaphotrapezial joint. We hypothesize that placement of the screw through the scaphoid tuberosity will achieve perpendicular fixation of an oblique waist fracture and result in more stable fixation than a screw in the center of the proximal fragment. Methods We designed oblique osteotomies for 8 matched pairs of cadaver scaphoids and fixed each specimen with a headless cannulated screw. In 1 specimen, we positioned the screw at the center of the proximal fragment; we placed its matched pair perpendicular to the fracture. The perpendicular screw was directed through the scaphoid tuberosity. We placed the specimen under the increasing load of a pneumatically driven plunger. We compared stiffness, load, distance at failure, and mechanism of failure between the central and perpendicular screw groups. Results We found no difference between groups. Stiffness was identical in both groups (131 N/mm) and load to failure was similar (central screw, 137 N vs perpendicular screw, 148 N). Conclusions In this biomechanical model of an unstable scaphoid fracture, we found that similar stability of fixation had been achieved with a screw perpendicular to the fracture plane with entry through the tuberosity, compared with a screw in a central position in the proximal fragment. This study suggests that placing the screw through the tuberosity, perpendicular to a short oblique fracture, will not impair fixation stability. Clinical relevance Percutaneous fixation of scaphoid fractures has become popular although it is technically challenging. An easier distal approach through the tuberosity, without violating the scaphotrapezial joint, may not impair the fixation stability of an oblique fracture. (J Hand Surg 2012;37A:1400–1404. Copyright © 2012 by the American Society for Surgery of the Hand. All rights reserved.) Key words Biomechanics, internal fixation, scaphoid fracture, wrist. HE OPTIMAL MODE of fixation of scaphoid fractures continues to be debated.1– 8 Percutaneous fixation of scaphoid fractures has become popular for treating undisplaced or minimally displaced fractures.9 More recently, others have used this technique for displaced fractures and nonunions.7
T
From the Departments of Orthopaedic Surgery and Medical Engineering, Hadassah–Hebrew University Medical Center, Kiryat Hadassah, Jerusalem, Israel; and Institute “Jožef Štefan” and the Department of Traumatology, University Clinical Center Ljubljana, Ljubljana, Slovenia. The authors acknowledge Synthes GmbH, Switzerland, for funding travel expenses, screws, and cadavers for this study. Received for publication December 5, 2011; accepted in revised form April 15, 2012.
1400 䉬 © ASSH 䉬 Published by Elsevier, Inc. All rights reserved.
Central placement of a headless cannulated screw in the proximal fragment of the scaphoid has become common practice and is considered superior biomechanically and clinically.1,10,11 Proper positioning of the screw in this position may be particularly demanding in a distal to proximal percutaneous technique.3,6 In No benefits in any form have been received or will be received related directly or indirectly to the subject of this article. Corresponding author: Shai Luria, MD, Department of Orthopaedic Surgery, Hadassah–Hebrew University Medical Center, Kiryat Hadassah, POB 12000, Jerusalem 91120, Israel; e-mail: [email protected]. 0363-5023/12/37A07-0015$36.00/0 http://dx.doi.org/10.1016/j.jhsa.2012.04.021
OPTIMAL FIXATION OF OBLIQUE SCAPHOID FRACTURE
an open distal approach, adequate positioning is possible through the scaphotrapezial joint. This positioning may be achieved with resection of the base of the trapezium or through the trapezium itself.12 Central placement of the screw in the proximal fragment has been evaluated on a cadaver model of transverse waist fractures without addressing other fracture configurations.1 Previously, a finite element analysis model showed that the most stable fixation may be achieved by placing a screw perpendicular to the fracture plane and not necessarily parallel to the long axis of the scaphoid. This difference was especially pronounced in an oblique fracture.5 The aim of this cadaver study was to compare fixation of an oblique scaphoid fracture using a screw perpendicular to the fracture, entering through the scaphoid tuberosity with a screw centrally located in the proximal fragment. Specifically, we aimed to determine whether a percutaneous screw placed through the tuberosity and perpendicular to a fracture could achieve adequate stability. Our global null hypothesis was that there would be no difference in stability between the 2 types of fixation. MATERIALS AND METHODS We removed 8 matched pairs of scaphoids from fresh cadaver wrists for comparison. Each pair consisted of a right and a left scaphoid from 1 individual. We performed an oblique osteotomy on each scaphoid and fixed it with a screw either perpendicular to the osteotomy and through the tuberosity or in the center of the proximal fragment. We applied load to the specimens to evaluate the stability of the fixation. The outcome variables were load at failure of fixation and mechanism of failure. Unstable fracture simulating osteotomy We cleaned the scaphoids of all soft tissue and determined the density of each scaphoid according to Archimedes’ principle (stating that the volume of displaced fluid is the volume of the object, measured as mass per volume). We randomly assigned the scaphoids within each pair to placement either central in the proximal fragment or perpendicular to the screw. For each matched pair, we fixed the complementary scaphoid with the opposite screw position, either central or perpendicular. We marked each specimen circumferentially about the narrowest aspect of the scaphoid waist, transverse to the general axis of the bone, using a 3-0 nylon thread as a guide. We marked the volar side of each scaphoid (marked by the tuberosity) 2 mm proximal to the midline mark, marked the dorsal (opposite)
1401
surface 2 mm distal to that line. We made a line between these 2 points on either side of the bone. We marked the osteotomy site on each specimen and planned the desired form of screw placement before we performed the osteotomy. Each scaphoid was placed in a vice, and a smooth oblique cut with a thin-blade, pneumatically driven circular bone saw created an oblique fracture.13 Two types of fixation of the simulated fracture After the osteotomy, we drilled a single Kirschner wire through each specimen according to the study group. We made high-resolution radiographs in the anteroposterior (AP) and lateral planes to confirm correct placement of the wire (Fig. 1). To standardize the radiographs taken, the tuberosity marked the volar aspect of the bone and the articulation surface with the capitate, the medial aspect. We defined central positional placement of the wire for the central screw as a wire position within the central one-third of the proximal fragment on both the AP and lateral radiographs.1 On each radiograph, we divided the section of the wire through the proximal fragment into thirds and evaluated the location of the wire in the central third of the bone at 2 equal points along this part of the wire (one-third height and twothirds height) (Fig. 1). At each of these points, the distance between the wire and cortices of the bone was calculated and expressed as a fraction of the entire width of bone at that location. This was expressed on a scale of 0 to 1, where 0 or 1 was a wire adjacent to the side of the bone and 0.5 was exactly central at this location. Using this method, the central third ranged between approximately 0.33 and 0.66. We defined radiographic evaluation of the wire for the perpendicular screw as placement that was perpendicular to the fracture and within the central one-third of the osteotomy plane on the AP and lateral views (Fig. 1). We measured this according to the same principle as described above, and expressed it as a fraction of the distance of the wire (crossing the fracture) to the cortices and the entire width of bone along the osteotomy line. After we placed the wire satisfactorily, we inserted the appropriately sized headless cannulated screw over the wire, according to the manufacturer’s guide (HCS 2.4; Synthes GmbH, Oberdorf, Switzerland). We inserted all screws to 2-finger tightness and made additional radiographs in the AP and lateral planes to confirm placement, and to verify that all proximal screw threads crossed the osteotomy.
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FIGURE 1: After cutting the oblique osteotomy, we placed a Kirschner wire to guide the cannulated screw. The left images are lateral and anteroposterior radiographs of the scaphoid with a perpendicular fixation; the right images are of the fixation in the center of the proximal fragment. Note the direction of the perpendicular wire toward the scaphoid tuberosity.
Testing of fixation stability For mechanical testing, we potted the proximal fragment of each specimen in a holder with polyurethane (Fig. 2). We passed a Kirschner wire through the proximal end of the scaphoid to provide additional anchoring stability before potting the specimen.1 The scaphoid was oriented at a 45° angle to the horizontal plane of the holder, to mimic its normal attitude in a wrist held in the neutral position. This enabled the delivery of a dorsalto-volar cantilever load, which represents the primary physiologic load encountered by the scaphoid.1,14 We then placed each specimen into a fixture placed on a load-cell (9281CA; Kistler Instrumente AG, Winterthur, Switzerland). A pneumatically driven plunger resting on the surface of the distal pole produced a load with a constantly increasing excursion at a rate of 0.014 mm/s. We measured the excursion of the plunger (DNCE; Festo, Esslingen, Germany) by a linear variable motor controller (CMMP-AS-C23A; Festo). We increased the load until the fixation failed by fracture and loss of reduction. We recorded load at failure and mechanisms of failure on a personal computer with data acquisition hardware and software (Visual C⫹⫹, Visual Studio; Microsoft,
Redmond, WA), via a Gateway sysWORKS (type CAN; Norton-Symantec, Mountain View, CA). We calculated stiffness from the load and displacement measurements. Estimation of sample size and power We calculated the sample sizes assuming equal sizes of both study samples, with ␣ ⫽ 0.05,  ⫽ 0.20 (for 80% power with 95% confidence), and D ⫽ 0.40 (for standard deviation). The sample size computed resulted in 8 pairs of scaphoids. This sample size was consistent with a previous power analysis performed using biomechanical testing data, which showed that 8 to 12 runs with each screw should provide significant data.1,15,16 Data analysis We used the Wilcoxon signed-rank nonparametric analysis to compare the stiffness and strength of the specimens. Statistical significance was established at P ⬎ .05. RESULTS We found no difference between the mean density of the left and right scaphoid pairs. The mean density was
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at the 2 points measured; in the lateral views, the mean was 0.49 (SD, 0.10) and 0.5 (SD, 0.75), with a range of 0.36 to 0.63 at the 2 points measured. Biomechanical testing demonstrated no significant difference between placements of the screw in the center of the proximal fragment of the scaphoid compared with a screw positioned perpendicular to the fracture plane. Load to failure of the fixation was 137 ⫾ 69 N for the central screw, compared with 148 ⫾ 49 N for the perpendicular screw. Displacement of fracture at failure was 1.2 ⫾ 0.5 mm for the central screw and 1.5 ⫾ 0.3 mm for the perpendicular screw, which was an insignificant difference. Stiffness was similar between groups (131 ⫾ 59 N/mm for the central screw vs 131 ⫾ 41 N/mm for the perpendicular screw). The mode of failure for all specimens was screw migration and fracture at the screw– bone interface, with the distal fragment sliding along the oblique fracture plane. None of the specimens dislodged from the potting fixture. Upon retrieval of the screws after failure, none were bent.
FIGURE 2: Schematic depiction of the potted scaphoid specimen and testing apparatus for delivery of a dorsal-tovolar cantilever bending load.
0.47 g/cm3 (SD, ⫾ 0.07) for the perpendicular fixed specimens and 0.47 g/cm3 (SD, ⫾ 0.05) for the centrally fixed specimens. Therefore, it was not necessary to normalize measurements of load and stiffness to bone density. We found the screw sizes used to be similar: 21.5 ⫾ 1.8 mm for the central screw and 21.5 ⫾ 2.6 mm for the perpendicular screw. Radiographic evaluation of all of the specimens adhered to the criteria described above for the placement of the screws (Fig. 1). The average angle between the fracture and wire was 4° (SD, 3°) on the AP view and 4° (SD, 3°) on the lateral view for the perpendicular fixation, and 19° (SD, 16°) on the AP view and 11° (SD, 11°) on the lateral view for the central fixation. In the perpendicular fixation group, all wires were directed through the central third of the fracture (mean, 0.5; SD, 0.1). In the central screw group, all wires passed through the central third of the proximal fragment of the specimen. The calculated location of the wires in the central third of the proximal fragment were as follows (0.5 is a perfect central location at the point measured): In the anteroposterior views, the mean was 0.46 (SD, 0.04) and 0.51 (SD, 0.05), with a range of 0.40 to 0.57
DISCUSSION The purpose of this study was to evaluate the biomechanical properties of a different approach to the fixation of acute scaphoid fractures. Our hypotheses were that a distal approach through the tuberosity and not through the scaphotrapezial joint will not impair fixation stability, and that a screw placed perpendicular to the fracture plane will result in improved stability compared with a screw placed in the center of the proximal fragment, as previously suggested.5,6 According to this study, fixation of a scaphoid fracture through the tuberosity and perpendicular to the fracture plane resulted in comparable stability to a screw placed in the center of the proximal fragment. Not only can this simplify the percutaneous distal fixation technique, it may also make it unnecessary to disturb the scaphotrapezial joint or resection of part of the trapezial base to achieve desirable screw positioning. Although we found that higher loads were necessary to displace the fracture with the perpendicular screw compared with a screw in the center of the proximal fragment, this was not significant statistically. A finite element model compared the biomechanical properties of the fixation of acute scaphoid fractures of different configurations with a screw placed perpendicular versus central in the proximal fracture fragment.5 This study reported that the perpendicular screw had a biomechanical advantage, and that this increased with the more oblique fractures. For this reason, we designed the current cadaver study with a short oblique unstable
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configuration, to accentuate the difference in location of the perpendicular and central screws. The design of this grossly unstable fracture resulted in lower loads to failure compared with previous studies. The loads measured in this study averaged 137 N for the central screw and 148 N for the perpendicular screw. McCallister et al1 evaluated more stable transverse fractures that resulted in loads between 500 and 700 N. Fulkerson et al17 resected a volar wedge in the scaphoid to simulate the humpback deformity of a fracture nonunion. This resulted in lower loads to displacement (approximately 200 – 400 N, measured at 4-mm displacement). A wedge of this shape is probably less stable than the simple transverse osteotomy. This cadaver study did not show an advantage to a perpendicular screw compared with the finite element model. There may be several reasons for the difference between the studies. The computerized model is superior in its precision compared with the cadaver model. It is possible that the insignificant difference in favor of the perpendicular fixation found in the cadaver study would be significant in a larger study group. In contrast, the finite element method is only as good as the factors taken into account in its design. While performing the cadaver study, we noticed that the central screw may be directed more parallel to the plunger used as the displacing force. In this situation, the screw itself may have contributed more resistance when it was placed in a more parallel direction, although we cannot prove this hypothesis. There are several limitations to this study, as with any cadaver study. We isolated the specimens of all surrounding tissue and evaluated only stability of fixation. We simplified the forces mimicking physiological forces into a single vector and did not perform cyclic loading. In addition, the linear osteotomy chosen does not mimic the comminution or interdigitation possible in an actual fracture, which affects its stability. According to the results of this cadaver model, a screw placed through the scaphoid tuberosity and perpendicular to the fracture will result in stability similar to that of a screw placed in the center of the proximal scaphoid that is aimed at the scaphotrapezial joint.
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