On the permanent hip-stabilizing effect of atmospheric pressure
Introduction
Primary total hip arthroplasty (THA) is among of the most frequent and successful procedures in present orthopedic surgery (Learmonth et al., 2007). Nevertheless, dislocations are still observed in 2–5% of all cases following primary THA and in 5–10% of cases following THA revision (Sanchez-Sotelo and Berry, 2001), especially occurring in the early postoperative course. THA dislocations are also the second most common cause of THA revision after aseptic loosening (Kohn et al., 1997) and often accompanied by further complications or increased morbidity. If we consider that in Germany alone, of the 214,000 THAs carried out in 2011, 4000–10,000 patients would suffer from THA dislocations, avoiding this complication clearly remains a major challenge (Morrey, 1992, Phillips et al., 2003, Sanchez-Sotelo and Berry, 2001). Several risk factors have been described (Sanchez-Sotelo and Berry, 2001), e.g. the surgical approach, misalignment of the components, a limited range of motion (ROM) and impingement. However, correction for these risk factors by ensuring alignment close to the initial physiological condition (Lewinnek et al., 1978) or THA with an increased ROM could not solve the problem of dislocations (Burroughs et al., 2005, Lewinnek et al., 1978). To minimize the risk of THA dislocations, we therefore need to identify further mechanisms that impact joint stability.
To date, most of the passive factors that have been reported to influence hip-joint stability are discontinuously active concerning the position of the joint partners. These factors encompass the accuracy in fit of the coxal bone with the femoral head in a sense of form closure (Fick, 1910, Platzer, 1979) and the ligaments of the hip joint capsule (Fuss and Bacher, 1991, Hewitt et al., 2001, Martin et al., 2008). Also, the effects of the muscles that surround the hip joint as active stabilizers (Flack et al., 2012) as a potential part of neuromuscular feedback mechanisms (Muratl et al., 2004) are discontinuous. In contrast, atmospheric pressure might have a permanent influence on the hip joint. The Weber brothers first showed in 1837 that a hip joint with a weight suspended from the femur dislocated if the ambient pressure was lowered (Weber and Weber, 1837). If ambient pressure was again increased, the hip joint subsequently reduced. Therefore, according to the theory of the Weber brothers, atmospheric pressure would permanently influence the stability of all joints independent of energy-consuming muscle contractions (Weber and Weber, 1836, Weber and Weber, 1837). Although this type of passive joint stabilization seems physiologically reasonable, discussions on this effect are ongoing until today (Prietzel et al., 2008).
Even though several authors have confirmed and expanded the Weber experiments (Aeby, 1876, Braam-Houckgeest, 1877, Fick, 1910), a number have argued that a hip-stabilizing effect of the atmospheric pressure would require a vacuum in the joint cavity and thus a tensile force acting on the joint (Buchner, 1877, Christen, 1911, Frick et al., 1980, Gerken, 1897). Nevertheless, there is a consensus that a vacuum only occurs under exceptional circumstances (Braam-Houckgeest, 1877, Fick, 1910, Frick et al., 1980, Gerken, 1897). Therefore, some authors have concluded that the effects of atmospheric pressure are irrelevant for hip joint stability (Buchner, 1877, Christen, 1911, Frick et al., 1980, Gerken, 1897).
In this study, we have used abstracted joint models with THA components to clarify the effect of atmospheric pressure on joint stability. We hypothesized that atmospheric pressure exerts a permanent hip-stabilizing effect and that an elimination of atmospheric pressure from the ambience of the joint models causes instability and even dislocation.
Section snippets
Methods
An abstracted model of the human hip joint was assembled from prosthetic components. Femoral heads (Biolox delta, made by CeramTec AG, Plochingen, Germany) with five different diameters were used (28, 32, 36, 40 and 44 mm). The heads were fixed conically on aluminum tapers (Fig. 1; Table 1). Modified prosthetic cups (ESKA Implants AG, Lübeck, Germany; Fig. 1) represented the acetabular components. The cups were made of polyethylene and consisted of an articulating surface identical to normal THA
Results
A total of 200 experiments were accomplished with the THA models. When the ambient pressure reached about 6.0 kPa, a subsequent dislocation of the femoral component was observed in the vertical direction. Table 1 shows the recorded pressure and displacement data.
In step 1, no motion or capsule deformation was observed. The intraarticular pressure was equal to ambient and atmospheric pressure (Fig. 4, Fig. 5). In step 2, initially no motion or capsule deformation was seen until the ambient and
Discussion
According to the theory concerning the permanent hip-stabilizing effect of atmospheric pressure, larger head sizes will considerably increase joint stability following THA. Applying this theory to practice, a lower risk of dislocation can be expected (Prietzel et al., 2007), which was confirmed in recent studies (Bistolfi et al., 2011, Howie et al., 2012). Furthermore, the effects of atmospheric pressure helped explaining the successful use of dual mobility or jumbo heads (Beaule et al., 2002,
Conclusions
Our experiments demonstrated the spontaneous dislocation of hip joint models, if the ambient pressure was lowered under 6.0 kPa. The onset of this phenomenon is prevented by the effects of atmospheric pressure, continuously stabilizing the hip joint. This effect of atmospheric pressure is temporarily supported by gravity, adhesion effects and by muscle activity. Consequently, using larger femoral heads in THA increases hip joint stability regardless of the ROM. Moreover the restoration of the
Conflict of interest statement
The authors report no conflicts of interest.
Acknowledgments
The authors would like to thank Angela Steller and Christine Auste for their help with the pictures. Klaus-Wolfram Richter and Michael Schmidt provided technical support. Gustav F. Preller assisted in proofreading the manuscript as a native speaker.
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