Are Tapered Titanium Stems With A Similar Geometry And Hydroxyapatite Coating Extension All The Same?

Background Several studies have shown that Corail stem provides high rates of satisfactory clinical results at long term follow-ups. Early concerns were possible complications related to such extensive HA coating, including HA resorption, delamination, osteolysis and early wear (1,2), as well as the risk of subsidence in collarless stems (3). However, long-term results have shown that the risk of coating-related complications is low (4-6), while stem subsidence may occur with different effects on the clinical outcome (7-10). In view of the long-term survival rate of the original stem, several tapered titanium femoral stem with extensive hydroxyapatite coating (TTSs-EHAC) have been introduced on the market in recent years. These stems resemble the original one in terms of geometry and extension of the HA coating, with no apparent differences between them. Objectives The purpose of this study was to compare the geometry of the original TTS-EHAC with that of 4 similar stems recently introduced; our hypothesis was that these new stems, despite appearing to duplicate the original one, actually have a different geometry that may affect their clinical indications. Study design and method: The coronal and sagittal geometry of the original Corail (CO) (DePuy, Johnson & Johnson, USA) and of four similar TTSs-EHAC, i.e. Trendhip (TH) (B.Braun-Aesculap, Germany), H-Max (HM) (Lima Corporate, Italy), Polarstem (PS) (Smith & Nephew, USA) and Avenir Muller (AM) (Zimmer, USA), were analysed (Fig.1). The assessment of the stem geometry included the measurement of the medio-lateral and antero-posterior diameters in the coronal and sagittal planes at the level of the proximal-medial extension of the HA coating, 10 mm proximally to the distal tip and in the middle between the two points (Fig 2). The total length of the stem, the length of the stem shoulder, defined as the tilted portion of the proximal and lateral side of the stem and the angle between a line tangent to the stem shoulder and the diaphyseal portion of the stem (meta-diaphyseal angle), were analysed (Fig. 2). The flare index (FI) of the stem was defined as the ratio between the coronal diameter of the stem measured at the level of the proximal-medial extension of the HA coating (A diameter, Fig. 2) and the coronal diameter measured 1 cm proximal to the tip of the stem (C diameter, Fig.2). The tapered index (TI) was calculated as the ratio between the cross-sectional area measured at the level of the proximal-medial extension of the HA coating and the cross-sectional area measured 1 cm proximal to the tip of the stem (Fig. 2). To assess any differences in stem geometry between brands, the coronal and sagittal diameters, flare indexes and tapered indexes were compared in all the sizes available for each brand as well as in the subgroup of comparable size. Measurements were performed on all the sizes of each brand on digitalized images of stem templates provided by the manufactures using AUTOCAD software Statistical analysis included Kruskal-Wallis and Mann-Whitney tests to detect any differences between the coronal and sagittal diameters, flare index and tapered index between stems of the five brands. Results: When all the sizes of the various brands were considered, a significant difference was found in the overall flare index and tapered index (p= 0.001 and p<0.001, respectively) while coronal and sagittal diameters, per se, were not different with the exception of the proximal coronal diameter which was significantly different in HM and AM (p=0.04). Comparable Sizes Six classes of sizes for each of the 5 brands (30 stems) yielded a comparable diameter in the middle portion of the stem (B diameter). The differences in B-diameter between the stems of each class of comparable sizes averaged 0.8 mm (range 0.5-1.3mm). A significant difference was found in the flare index (p=0.01) and tapered index (p=0.002) of comparable sizes between the different brands (Fig. 3). A significant difference was detected between the 5 brands in the length of stem shoulder (p=0.04). The meta-diaphyseal angle ranged between 163° and 171° (p<0.001). Conclusion: Several TTSs-EHC have recently been introduced on the market. Their geometry is similar and includes a tapered shape with a quadrangular cross section and a proximal flared portion in both planes. Horizontal and vertical groves are present on the porous coating, which is entirely covered with HA to enhance mechanical stability. As these stems closely resemble each other and no investigation has previously been conducted to detect any differences between them, it might be assumed that any one of these stems can be used indifferently whenever a cementless stem is indicated. Our results have shown that the flare index and tapered index yielded a significant difference when all the brands were considered; when each stem was matched with the others, the difference in flare index was still significant between HM and all the remaining stems except PS. The tapered index, which takes into account the ratio between the proximal and distal cross-sectional areas of the stem, differed significantly both when all brands were analysed together and when each stem was compared with the others. As the number of sizes of the five brands ranges between 8 and 11, to compare stems of similar size, we selected 30 stems, 6 for each brand, with a similar coronal diameter in the middle portion of the stem. The analysis of these 30 stems showed that both the flare index and tapered indexes differ significantly among the 5 stems analysed. These findings indicate that some of these stems exhibit a significantly more pronounced funnel shape than others, thus suggesting that their biomechanical behavior may vary in different morphologies of the proximal femur. We suggest that these stems are not used indifferently in any patient in whom a cementless stem is indicated but rather on the basis of a pre-operative planning in which the stem showing the best fit and fill is selected.