Deformation of acetabular press-fit cups: Influence of design and surgical factors

Abstract

Background: Deformation of acetabular cups when press-fitted into an undersized cavity is inevitable due to the inhomogeneous stiffness of acetabular bone. Thinner cups or screw holes might increase the risk of high cup deformation. The aim of this study was to examine the influence of cup design and liner assembly on the deformation response during cup implantation. Methods: Acetabular cups with different designs were implanted into polyurethane foam models simulating the anatomical situation with nominal press-fits of 1mm and without nominal press-fits (line-toline). Deformations were determined using a tactile coordinate measuring machine. A 3D laser scanner was used to determine the contact conditions at the cup-cavity interface. Polyethylene and ceramic liners were assembled to the implanted cups and the influence of the insertion on the deformation response evaluated. Fixation strength of the cups was determined by push-out testing. Findings: Cup deformation increased with smaller wall thickness (P<0.037) and screw holes (P<0.001). Insertion of ceramic liners reduced the deformation (P<0.001), whereas polyethylene liners adapted to the deformation of the implanted cups (P>0.999). Thin-walled cups exhibited a higher fixation strength for similar implantation forces (P=0.011). Interpretation: Thin-walled cups achieved higher fixation strengths and might be more bone-preserving. However, in combination with screw holes and high press-fit levels, wall thickness should be considered carefully to avoid excessive cup deformations leading to potential complications during liner assembly. Line-to-line insertion of thin-walled cups should be accompanied with a rough surface coating to minimize the loss of fixation strength due to the low press-fit fixation.

Influence of acetabular cup thickness on seating and primary stability in total hip arthroplasty

“…This coincides with the higher deflections due to the lower cup stiffness of the thin-walled cups 19,33 and results in lower contact stresses to the bone. The previously demonstrated correlation of cup deflection and lever out moment 18 was in the present study only demonstrated for the thick-walled design. The thin-walled cups were found to be more robust exhibiting a rather constant high primary stability.…”

“…The insertion of hard liners can lead to a significant reduction in deformation. 18 High cup deformations do not necessarily lead to improper liner seating or excessive liner deformation but certainly, make the liner insertion more difficult. A detailed analysis of the time-dependent cup deformation could also help to assess whether liner insertion may benefit from a specific relaxation time.…”

“…In synthetic bone models, these high deformations occur already for small impaction forces. [17][18][19][20] Besides patient-and implant-related factors, the surgeon plays a crucial role for the success of total hip arthroplasty (THA). 2,21 Clinically, impaction is usually performed with a mallet, which allows for a wide range of variations.…”

Titanium Acetabular Component Deformation under Cyclic Loading

“…Multi-hole cups have been shown to present more deformation than single-hole cups during implantation [16], which allows one to assume that a similar effect may be seen under dynamic loading after implantation. For the purpose of comparability, a multi-hole cup was used in both scenarios, although this type of cup would tend not to be used clinically in a scenario without a bony defect.…”

“…Prior studies have focused on assessing the pattern and degree of deformation resulting from the implantation process [7,[9][10][11][12][13][14]. A few studies have assessed deformation after press-fit implantation and load application [15,16], however, these studies did not investigate cup deformation during load on the cup, as would be the case once the patient moves his/her hip.…”