Do Capsular Pressure and Implant Motion Interact to Cause High Pressure in the Periprosthetic Bone in Total Hip Replacement?

Abstract

When there is a debonding at the bone-implant interface, the difference in stiffness between the implant and the bone can result in micromotion, allowing existing gaps to open further or new gaps to be created during physiological loading. It has been suggested that periprosthetic fluid flow and high pressure may play an important role in osteolysis development in the proximity of these gaps. To explain this phenomenon, the concepts of "effective joint space" and "pumping stem" have been cited in many studies. However, there is no clear understanding of the factors causing, or contributing to, these mechanisms. It is likely that capsular pressure, gap dimensions, and micromotion of the gap during cyclic loading of an implant can play a defining role in inducing periprosthetic flow. In order to obtain a better understanding of the main influences on periprosthetic flows and the development of osteolysis, steady state and transient 2D computational fluid dynamic simulations were performed for the joint capsule of the lateral side of a stem-femur system, and a gap in communication with the capsule and the surrounding bone. It was shown that high capsular pressure may be the main driving force for high fluid pressure and flow in the bone surrounding the gap, while micromotion of only very long and narrow gaps can cause significant pressure and flow in the bone. At low capsular pressure, micromotion induced large flows in the gap region; however, the flow in the bone tissue was almost unaffected. The results also revealed the existence of high velocity spikes in the bone region at the bottom of the gap. These velocity spikes can exert excessive fluid shear stress on the bone cells and disturb the local biological balance of the surrounding interstitial fluid which can result in osteolysis development. High capsular pressure was observed to be the main cause of these velocity spikes whereas, at low capsular pressure, gap micromotion of only very long and narrow gaps generated significant velocity spikes in the bone at the bottom of the gaps.

Emerging Ideas: Instability-induced Periprosthetic Osteolysis Is Not Dependent on the Fibrous Tissue Interface

“…2). Pressures generated in the instability-induced model previously were measured using a pressure transducer, and are similar to those reported for loose hip implants in vivo [1,7,20]. The loading device never contacts the underlying cortical bone in this model.…”

“…To our knowledge, this is the only animal model mimicking the mechanical situation around an implant. Other studies focusing on physical factors involved in osteoclast differentiation have been performed in vitro [1,5]. However, in vitro experiments are limited by the ability to extrapolate their findings to the in vivo environment.…”

“…However, studies have shown that hydrostatic pressure and fluid flow of a clinically relevant magnitude, transmitted through a fibrous tissue membrane but in the absence of wear particles and debris, may induce the formation of periprosthetic osteolytic lesions [2, 3, 5-7, 11, 23, 25]. The pressurized fluid flow velocity along the bone-implant interface has been estimated [7] and calculated [1] to be 10 times larger than that during normal physiologic loading. Elevated pressures, as much as 155 to 776 mm Hg, can be present in patients with loose hip implants before revision surgery during different mechanical maneuvers [20].…”

Micromotion-induced peri-prosthetic fluid flow around a cementless femoral stem

“…The micromotion-induced peak fluid velocity at the bone-cement interface of retrieved transverse sections of cemented femoral stems varied from 270 to 15700 μm/s (Mann & Miller 2014). In another model of capsular pressure and micromotion-induced fluid flow around a cementless femoral stem, fluid velocities extending up to 3000 μm/s were observed (Alidousti et al 2011). In a magnetic resonance imaging study, micromotion-induced fluid velocity in the gap around a canine bone implant model reached 14000 μm/s (Conroy et al 2006).…”

“…Various studies in the recent years tried to characterize micromotioninduced fluid flow (Conroy et al 2006;Alidousti et al 2011;Mann & Miller 2014). Most of these studies considered simplified bone and implant geometries, unidirectional homogeneous micromotion, or were limited to 2D fluid velocities.…”