In this study, we proposed a more detailed fracture classification based on whether the oblique fracture line mainly crosses the horizontal or vertical plane of the metacarpal shaft. Our results revealed that the fixation strength, measured by determining the effectiveness in sustaining force and stiffness, did not significantly differ among the LS, RP, and LP fixation. Treatment of these two types of oblique fractures differs as follows: (1) When dorsal bone plate fixation is used to fix type II fractures, the screw on the bone plate can serve as a lag screw, thus increasing the stability of the fracture end (Fig. 5 right). In type I fractures, the screw on the bone plate cannot serve as a lag screw. Therefore, the effectiveness of using bone plates to fix type I fractures is poorer (Fig. 5 left). (2) During hand prehension, the intrinsic muscles of the hand generate a bending force toward the metacarpal bone. Therefore, for type I fractures, the vector of the bending force can result in severe fracture site displacement. By contrast, for type II fractures, the vector of the bending force exerts a weaker effect in terms of causing fracture site displacement.
Metacarpal fractures are often caused by a direct blow during violence, axial loading due to falls on an outstretched hand, and torsion force from forceful traction [18]. Different injury mechanisms result in different fracture types. Fractures are typically categorized as transverse, oblique, spiral, or comminuted [18]. Vertical force and direct impact cause transverse or comminuted fractures, whereas torsion force results in oblique or spiral fractures. Among the different types of fracture, oblique and spiral are the most common, accounting for approximately 75% of all fractures [18]. The bone contact surface of the fracture site is small in transverse fractures. Thus, such fractures often result in nonunion due to the displacement or overlapping of the fractured bone end [4, 8]. This type of fracture usually requires surgical intervention. According to the literature and our previous mechanical study, favorable outcomes (fixation strength) can be achieved using both bone plate and intramedullary screw fixation [19,20,21]. The use of lag screws in transverse bone fractures is not favorable because they do not result in satisfactory fixation strength [22]. When bone plate fixation is performed to treat oblique fractures, bone plates of longer lengths are required to cover the longer fracture zone and ensure that screws can be fixed on the uninjured bone end. This is associated with extensive soft tissue dissection and postsurgical complications, including tendon adhesion, scar contracture, and joint stiffness [23]. By contrast, the use of lag screws to treat oblique metacarpal shaft fractures has resulted in favorable outcomes and does not require extensive soft tissue dissection around the fracture site [19, 20]. Treatment is generally effective when lag screw fixation is performed. However, biomechanical studies on lag screw fixation, especially those comparing the effectiveness of lag screw fixation with that of bone plate fixation in treating oblique metacarpal shaft fractures, are rare. Therefore, we proposed a more detailed fracture classification based on the fracture pattern and compared the fracture fixation effectiveness of lag screws and bone plates by using this classification.
In the studies on metacarpal fracture fixation, Chiu et al. [19, 24] used the same artificial metacarpal bone and cantilever bending biomechanical test. They used the headless compression screw, plate, and regular plate to fix the fracture in the middle of the metacarpal diaphysis. For the three fixation methods, the maximum fracture force values were 285.6, 227.8, and 228.2 N, respectively, and the stiffness values were 65.2, 61.7, and 54.9 N/mm, respectively. In addition, Chiu et al. [25] indicated that the use of the lag screw, regular bone plate, and locking plate to fix metacarpal vertical oblique shaft fractures resulted in maximum fracture force values of 153.6, 211.6, and 227.5 N, respectively, and stiffness values of 57.0, 64.7, and 65.4 N/mm, respectively. The findings of the previous study [25] and those of the current study indicate that the fixation strength of lag screws is not inferior to that of a metallic plate in either horizontal or vertical oblique fractures.
We used stiffness as an indicator to determine the fixation strength. However, we did not adopt the maximum fracture force as an indicator because when the material testing system was used to apply force on the distal region of the dorsal side of the artificial metacarpal bone in the RP and LP fixation, the maximum force was not observed at the actual fracture site in the entire specimen, as determined by plotting the force–displacement curve. Instead, the maximum force was observed at the point where the fixture on which force was applied slipped off the artificial metacarpal bone (Fig. 6a). At this time, the bone plate was already inserted into the plastic deformation region. Therefore, for the RP and LP fixation, the force value corresponding to the yield point was considered appropriate as the evaluation index, and we used it to represent the failure force. In the LS fixation, the samples exhibited an actual fracture and loosening, as determined from the force–displacement curve (Fig. 6b).
The surgical methods most commonly used in clinical practice to treat oblique metacarpal shaft fractures are: (1) lag screw fixation, (2) bone plate fixation, and (3) K-wire fixation [6]. Among these methods, bone plate fixation is generally considered to result in the strongest fixation effect; however, it requires longer surgical incisions and more extensive soft tissue dissection and is more expensive [9, 26]. For K-wire fixation, the fixation strength is insufficient to withstand the torsion load at the fracture site; this can lead to the rotational malunion of the fracture and eventually a scissoring deformity [2]. Therefore, most hand surgeons do not consider K-wire fixation to be suitable for the treatment of oblique metacarpal fractures. Lag screw fixation is a less invasive surgical procedure and typically results in favorable outcomes [27]. However, the uncertain mechanical stability of fixing by using only lag screws and the precise surgical technique required to place lag screws are major concerns for surgeons [27]. However, because the bone contact area of oblique metacarpal shaft fractures is relatively large, the bone healing potential for such fractures is higher than that for transverse fractures. Lag screw fixation can be more favorable for a larger fracture zone in oblique fractures. In addition to not causing complications normally associated with bone plate fixation, lag screw fixation involves shorter surgical incisions, less soft tissue dissection, lower costs, and shorter operating times [19, 20, 28].
This study has several limitations. First, we used artificial bone instead of human bone because of difficulty procuring fresh human metacarpal bones. Furthermore, even if such fresh bones were obtained, ensuring that all specimens possessed similar material properties would be impossible. Therefore, similar to most studies [1, 7, 20, 29], we used artificial bone instead of human metacarpal bone. Second, we performed the cantilever bending test to evaluate the effectiveness of different fixation methods for horizontal oblique metacarpal shaft fractures; this method was similarly used in other studies [19, 20, 30]. However, the movement of this loading model differs from that of real hands. Therefore, additional comprehensive experiments must be conducted to gain a better understanding of this topic. Third, compared with a true oblique fracture, a spiral-type oblique fracture is more commonly encountered in a real clinical scenario. However, in a biomechanical study, a true oblique fracture can be more easily reproduced and standardized in artificial metacarpal bone, and we can obtain study results with high reliability. Although we examined true oblique fractures in this study, we will focus on spiral-type oblique fractures in our future study.
