Geometry of human ribs pertinent to orthopedic chest-wall reconstruction

https://doi.org/10.1016/j.jbiomech.2006.05.017Get rights and content

Abstract

Orthopedic reconstruction of blunt chest trauma can aid restoration of pulmonary function to reduce the mortality associated with serial rib fractures and flail chest injuries. Contemporary chest wall reconstruction requires contouring of generic plates to the complex surface geometry of ribs. This study established a biometric foundation to generate specialized, anatomically contoured osteosynthesis hardware for rib fracture fixation. On human cadaveric ribs three through nine, the surface geometry pertinent to anatomically conforming osteosynthesis plates was characterized by quantifying the apparent rib curvature CA, the longitudinal twist αLT along the diaphysis, and the unrolled curvature CU. In addition, the rib cross-sectional geometry pertinent to intramedullary fixation strategies was characterized in terms of cross-section height, width, area, and cortex thickness. The rib surface exhibited a curvature CA ranging from 3.8 m−1 in the anteromedial section of rib seven to 17.3 m−1 in the posterior section of rib three. All ribs had in common a longitudinal twist αLT, ranging from 41–60°. The unrolled curvature CU decreased gradually from ribs three to five, and increased gradually with reversed orientation from rib six to nine. The cross-sectional area remained constant along the rib diaphysis. However, the medullary canal increased in size from 29.9 mm2 posteriorly to 41.2 mm2 in anterior rib segments. Results of this biometric rib characterization describe a novel strategy for intraoperative plate contouring and provide a foundation for the development of specialized rib osteosynthesis strategies.

Introduction

Rib fractures are present in 4–10% of trauma patients admitted to hospitals (Mayberry and Trunkey, 1997). About 10% of chest traumata result in a flail chest (Lardinois et al., 2001), defined as segmental facture of at least four consecutive ribs. Flail chest injury is associated with a mortality rate of 10–36% (Cacchione et al., 2000; Hellberg et al., 1981; Labitzke, 1981; Landercasper et al., 1984; Lardinois et al., 2001; Schmit-Neuerburg et al., 1982). In flail chest injuries, paradoxical inward movement of the flail segment prevents effective inspiration and requires prolonged mechanical ventilation, which in turn can lead to pneumonia and sepsis (Labitzke, 1981; Lardinois et al., 2001; Mouton et al., 1997; Tanaka et al., 2002; Tscharner et al., 1989).

Operative fixation of multiple segmental rib fractures can accelerate restoration of pulmonary function (Ahmed and Mohyuddin, 1995; Haasler, 1990; Labitzke et al., 1980; Tanaka et al., 2002; Tscharner et al., 1989; Voggenreiter et al., 1998; Voggenreiter et al., 1996), shorten ICU stay and hospitalization (Ahmed and Mohyuddin, 1995; Haasler, 1990; Tanaka et al., 2002), and reduce mortality associated with prolonged mechanical ventilation (Balci et al., 2004; Karev, 1997; Labitzke, 1981; Landreneau et al., 1991; Lardinois et al., 2001; Quell and Vecsei, 1991; Tanaka et al., 2002; Tscharner et al., 1989; Velmahos et al., 2002). A further benefit of operative chest wall stabilization is a decreased likelihood of clinically significant long-term respiratory dysfunction and skeletal deformity (Haasler, 1990; Meier and Schuepbach, 1978).

Despite the potentially live-saving intervention, no specialized hardware for rib reconstruction is commercially available today. Early plate designs for rib fixation were straight and required intraoperative contouring (Borrely et al., 1985; Martin et al., 1982; Menard et al., 1983; Vecsei et al., 1979). Today, generic small fracture plates are most commonly used for rib fracture fixation (Engel et al., 2005; Friedrich et al., 1991; Lardinois et al., 2001; Mouton et al., 1997; Ng et al., 2001; Oyarzun et al., 1998; Reber et al., 1993). However, due to the complex geometry of ribs, intraoperative contouring of generic plates is time-consuming and difficult at best. Furthermore, the use of generic plates with vastly varying mechanical and geometric properties is accompanied by a high incidence of screw loosening and pullout (Boetsch and Rehm, 1981; Friedrich et al., 1991; Hellberg et al., 1981; Mouton et al., 1997; Voggenreiter et al., 1996). As holds true for precontoured osteosynthesis plates in general, anatomically conforming rib plates will likely improve anatomic reduction and stability, significantly shorten the operation time, and consequently decrease the risk of infection.

As a less-invasive alternative to plate osteosynthesis, Kirschner wires for intramedullary fixation have been used (Quell and Vecsei, 1991). Intramedullary fixation is especially attractive for posterior rib fractures that are not amenable for plate fixation due to lack of surgical accessibility. However, the circular cross-section of Kirschner wires provides little rotational stability and lack of secure fixation inside the canal can lead to wire migration (Ahmed and Mohyuddin, 1995; Albrecht and Brug, 1979; Meier and Schuepbach, 1978; Moore, 1975; Vecsei et al., 1979). Akin to standard practice for intramedullary fixation, knowledge of the medullary canal geometry in ribs will be crucial to select a properly dimensioned intramedullary fixation device.

Most recently, Engel et al. (2005) concluded that further research is necessary to design specific fixation hardware which takes into account the structural properties, geometry, and fixation constraints of ribs.

While rib cage dimensions have been obtained with several techniques (Fick et al., 1911; Fujimoto et al., 1984; Nussbaum and Chaffin, 1996), a comprehensive description of the geometric properties of individual ribs is not available to date. Most pertinent to plate fixation, no study has described the barrel-shaped outer surface of the human rib cage to which plates have to be contoured intraoperatively. The cross-sectional geometry of human ribs has been quantified for the mid-diaphyseal region only, without examining changes thereof along the rib (Stein and Granik, 1976; Takahashi and Frost, 1966; Yoganandan and Pintar, 1998).

This study quantified the longitudinal geometry of human ribs in terms of their spatial surface geometry in order to aid development of anatomically contoured plating hardware. In addition, the cross-sectional geometry along ribs was quantified as a morphometric basis for the advancement of intramedullary fixation alternatives.

Section snippets

Method

This laboratory study evaluated the geometry of human cadaveric ribs three through nine. The first part of this study examined the longitudinal geometry of ribs by quantifying the apparent rib curvature, the longitudinal twist along the diaphysis, and the unrolled curvature of the outer cortical surface. The second part of this study investigated the cross-sectional geometry of ribs over their length by measuring the height and width of each cross-section, the cortical thickness, and the area

Longitudinal rib geometry

The apparent rib curvature CA varied along the length of individual ribs and between ribs of different anatomical levels (Fig. 3). Among ribs three through nine, CA was most consistent at 15% (CA,15%=15.2–17.3 m−1) and 85% (CA,85%=6.2–8.0 m−1). Among all ribs, the highest curvature was found posterior at 15% rib length, with rib three exhibiting the highest overall curvature of CA,15%=17.3±1.7 m−1. The curvature of ribs three and four decreased from posterior to anterior and was on average

Discussion

The first part of our study concerns the longitudinal geometry of the rib, which so far has mainly been derived from planar projections by the best fitting arc in each rib (Roberts and Chen, 1972; Schultz et al., 1974a, Schultz et al., 1974b; Wilson et al., 1987). However, as the curvature of the rib changes over length, this approach provides only rough descriptions of the actual geometry. Schultz et al., 1974a, Schultz et al., 1974b measured ribs two, four, six, and 8–10 of five male cadavers

Acknowledgment

Financial support for this research has been provided by the Legacy Research Foundation.

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