Objective: The adequacy of the cement mantle around various designs of impaction-grafted stems has been compared and deemed inadequate around the Exeter system. Yet, good clinical results have been reported. The conventional wisdom of solid cement mantles has been also been questioned in recent reports by the low migration and high survival rates of stems inserted with a very thin cement mantle – the so called ‘French paradox’. We performed this study specifically to address two questions

Does cement mantle thickness affect cement penetration depth during impaction grafting? and

Materials and Methods: 12 composite femurs were prepared to mimic cavitary defect. Impaction grafting was done with morcellized freshly frozen porcine femoral condyles using Exeter X-change system. The size of tamp and prosthesis were independently varied creating tamp/stem mismatch to produce cement mantles with a nominal thickness of 0, 1, 2, 3 or 4 mm. Cyclical loading was done at 1 Hz for 2500 cycles at 2500 N. From the displacement data measured by 6 linear displacement transducers we calculated subsidence and retroversion. The solid cement mantle and the penetration depth into the graft were then measured along 16 points in each cut section of the femurs done at 1.5 cm intervals.

Results: There was a high correlation between tamp/stem mismatch (nominal mantle thickness) and actual mantle thickness (r=0.84). Average cement penetration into the graft for each prosthesis varied between 0.3 and 2.0 mm. Largest variations were proximally, where average penetration varied between 0.4 and 3.5 mm. A thicker solid cement mantle gave on average less cement penetration (r=−0.62). Stem subsidence after cyclic loading ranged from 0.4 to 2.5 mm and correlated significantly with tamp size (r=0.59, p< .05). However, better correlations were found with solid mantle thickness (r=0.90, p< 0.05) and cement penetration depth (r=−0.81). Stem retroversion after cyclic loading ranged from 0.1 to 2.0 degrees and correlated negatively with stem size (r=−0.53) but did not correlate with tamp size. Correlations with solid mantle thickness and cement penetration depth were not better than those with tamp size.

Discussion: Our study shows that a thinner mantle is associated with deeper cement penetration into the graft. This probably is due to the higher cement pressure generated during stem insertion when there is less space for the cement to escape. Better mechanical interlock with the higher cement penetration possibly explains the reduced subsidence with thin cement mantles. Our study also shows that stem retroversion is associated with stem size only, and is larger for thinner stems. This could be explained by thinner stems providing less resistance to torsional forces.

The adequacy of cement mantles around some impaction-grafting systems has been criticised yet good clinical results have been reported. This study investigates this contradiction by asking

Does cement mantle thickness affect cement penetration depth?

Twelve artificial femora were prepared to simulate cavitary defects. Porcine cancellous bone was morselized. The defect was reconstructed by impaction grafting, using a size 0, 1 or 2 tamp. Bone cement was injected, and a size 0, 1 or 2 Exeter stem inserted. By using all nine tamp/prosthesis combinations, 0–4 mm thick cement mantles were produced. Femora were positioned in a testing machine and loaded with 2500 cycles of 2500 N. Prosthesis subsidence and retroversion were measured. Each femur was sliced transversely and the sections digitised. Solid cement mantle thickness and cement penetration depth were measured using image analysis. Correlation analysis was used to find if tamp/stem mismatch (nominal mantle thickness) influenced actual solid mantle thickness and cement penetration. We then analysed if tamp size, stem size, solid mantle thickness or cement penetration determined stem subsidence and retroversion.

Cement mantles were produced with an average thickness of 1.7–2.2 mm, with largest variations proximally (1.5–2.8 mm). Average cement penetration was 0.3–2.0 mm, with largest variations proximally (0.4–3.5 mm). Thicker solid mantles gave less penetration (r=−0.62). Stem subsidence ranged from 0.4–2.5 mm and correlated significantly with tamp size (r=0.59, p< .05). Better correlations were found with solid mantle thickness (r=0.90, p< 0.05) and cement penetration depth (r=−0.81). Stem retroversion ranged from 0.1–2.0 degrees and correlated with stem size (r=−0.53) but not with tamp size.

Tamp/stem mismatch determined the thickness of the solid cement mantle around impaction-grafted stems, and thinner mantles were associated with deeper cement penetration. Thinner mantles and deeper penetration were associated with reduced stem subsidence. Stem retroversion was associated with stem size only, and larger for thinner stems. Thinner cement mantles will therefore be associated with deeper penetration and reduced stem subsidence upon loading. This association may explain the good long-term results of impaction-grafted Exeter stems, despite deficient solid cement mantles.

Purpose: To investigate whether cement mantle thickness influence early migration of the stem after impaction grafting

Methods: Twelve artificial femora were prepared to mimic cavitary defects. After compacting morselized bone into the cavities, Exeter stems were cemented in place. By using all combinations of three sizes tamps and stems (0, 1 and 2), we created cement mantles of 0, 1, 2, 3 and 4 mm thickness. Bones with stems were placed in a testing machine and loaded cyclically to 2,500 N while measuring stem migration. Statistical analysis was by regression analysis. Outcomes were stem subsidence and retroversion, predictors were mantle thickness, tamp size and stem size.

Results: Average stem subsidence after 2500 cycles when using size 1 tamp and stem (2 mm mantle) was 0.94 mm. Cement mantle thickness significantly influ-enced stem subsidence (r=0.68, p=0.015). For a 0 mm mantle, subsidence was 0.59 mm and for a 4 mm mantle it was 2.54 mm. Cement mantle thickness also signifi-cantly influenced stem retroversion (r=0.62, p=0.031). Cement mantle thickness was a better predictor than tamp or stem size.

Discussion: Concern exists that inadequate cement mantles may affect stability of impaction-grafted stems. In our study, larger difference between tamps and stems gave substantially more subsidence and rotation, whereas a smaller difference reduced them. Concerns over thin mantles may have been premature.