Sufficient primary stability of the acetabular cup is essential for stable osseous integration of the implant after total hip arthroplasty. By means of under-reaming the cavities press-fit cups gain their primary stability in the acetabular bone stock. These metal-backed cups are inserted intra-operatively using an impact hammer.

The aim of this experimental study was to obtain the forces exerted by the hammer both in-vivo and in-vitro as well as to determine the resulting primary stability of the cups in-vitro.

Two different artificial bone models were applied to simulate osteoporotic and sclerotic bone. Polymeth-acrylamid (PMI, ROHACELL 110 IG, Gaugler & Lutz, Germany) was used as an osteoporotic bone substitute, whereas a composite model made of a PMI-Block and a 4 mm thick (cortical) Polyvinyl chloride (PVC) layer (AIREX C70.200, Gaugler & Lutz, Germany) was deployed to simulate sclerotic bone. In all artificial bone blocks cavities were reamed for a press-fit cup (Trident PSL, Size 56mm, Stryker, USA) using the original surgical instrument. The impactor of the cup was equipped with a piezoelectric ring sensor (PCB Piezotronics, Germany). Using the standard surgical hammer (1.2kg) the acetabular cups were implanted into the bone substitute material by a male (95kg) and a female (75kg) surgeon. Subsequently, primary stability of the implant (n=5) was determined in a pull-out test setup using a universal testing machine (Z050, Ziwck/Roell, Germany).

For validation the impaction forces were recorded intra-operatively using the identical press-fit cup design.

An average impaction force of 4.5±0.6kN and 6.3±0.4kN using the PMI and the composite bone models respectively were achieved by the female surgeon in vitro.

7.4±1.5kN and 7.7±0.8kN respectively were obtained by the male surgeon who reached an average in-vivo impaction force of 7.5±1.6kN.

Using the PMI-model a pull-out force of 298±72N and 201±112N were determined for the female and male surgeons respectively. However, using the composite bone model approximately half the pull-out force was measured for the female surgeon (402±39N) compared to the male surgeon (869±208N).

Our results show that impact forces measured in-vitro correspond to the data recorded in-vivo. Using the osteoporotic bone model the pull-out test revealed that too high impaction forces affect the pull-out force negatively and hence the primary implant stability is reduced, whereas higher impact forces improve primary stability considerably in the sclerotic bone model. In conclusion, the amount of impaction force contributes to the quality of the obtained primary cup stability substantially and should be adjusted intra-operatively according to the bone quality of each individual patient.

Introduction: To obtain secondary implant stability of acetabular press-fit cups, sufficient primary stability is essential. The aim of this study was to investigate the influence of cup insertion force and bone quality on the primary implant stability.

Materials and Methods: The experiments were carried out using two commercially available press-fit acetabular cups (Trident PSL, Stryker und EP-FIT PLUS, PLUS Ortho-peadics), comparable in design and with identical diameters, which were inserted axially into artificial bone by a female and a male surgeon. Two bone substitute material models were used. To imitate osteoporotic bone, a PMI-model (ROHACELL 110 IG, Gaugler & Lutz oHG) was employed. To simulate sclerotic bone, a composite-model made of a PMI-bloc with a 4 mm thick PVC-layer (AIREX C70.200, Gaugler & Lutz oHG) was used. The cups were inserted using an insertion device, equipped with a force sensor, and an 1100 g surgical hammer. Additionally, all experiments were carried out using a dynamic testing machine (25 kN, Instron) utilising insertion forces of 4.0 kN and 8.0 kN respectively. Primary implant stability was determined via lever-out tests using a static universal testing machine (Z050, Zwick/Roell).

Results: On average an insertion force of 4.8 kN (female) and 7.0 kN (male) using the PMI-model and 6.2 kN (female) and 7.5 kN (male) for the composite-model was assessed for the two different surgeons. The machined forces averaged 3.8 kN and 7.9 kN.

Lever-out-moments of 17 Nm were determined for both the PMI- and composite-model for the female surgeon using the PSL cup, whereas 27 Nm and 70 Nm, respectively, were reached for the EP-FIT shell.

For the male surgeon using the PSL cup, lever-out moments of 15 Nm and 30 Nm for the PMI- and composite-model respectively were determined. Insertion of the EP-FIT cup resulted in lever-out moments of 10 Nm using the PMI-model and 82 Nm using the composite-model.

The low machined insertion force led to average lever-out moments of 34 Nm for the PSL and 71 Nm for the EP-FIT cups using the composite-model. For the high machined force, the highest lever-out moments of 44 Nm and 99 Nm for the PSL and EP-FIT shells respectively were determined.

Conclusion: Using the composite-model (sclerotic bone), higher insertion forces lead to higher lever-out moments and hence higher primary implant stability for both tested cups. However, a high, non axial applied force can result in loss of stability using the PMI-model (osteoprotic bone). Compared to the manually inserted acetabular cups, the machined insertion resulted in higher primary stability for both implants and artificial bone types.