Trabecular Metal Technology Alexandre Nehme Departement of Orthopedic Surgery. Saint Georges University Hospital. Beirut. The cellular structure of Tantallum (Trabecular Metal) resembles bone and approximates its physical and mechanical properties more closely than other prosthetic materials. The unique, highly porous, trabecular configuration is conducive to bone formation, enabling rapid and extensive tissue infiltration and strong attachment. 1,2,+ Physical Properties Trabecular Metal consists of interconnecting pores resulting in a structural biomaterial that is 80% porous, allowing approximately 2-3 times greater bone ingrowth compared to conventional porous coatings and double the interface shear strength. 1,+ Trabecular Metal implants are fabricated using elemental tantalum metal and vapor deposition techniques that create a metallic strut configuration similar to trabecular bone. The crystalline microtexture of a Trabecular Metal strut is conductive to direct bone apposition. 2 Elemental tantalum unites strength and corrosion resistance with excellent biocompatibility. These characteristics help explain tantalum's surgical use for more than 50 years in applications such as cranioplasty plates and pacemaker leads. 3
Mechanical Properties Trabecular Metal possesses a high strength-to-weight ratio, with mechanical properties capable of withstanding physiologic loading. The compressive strength and elastic modulus of Trabecular Metal are more similar to bone than are other prosthetic loadbearing materials. 2,4 The material's low stiffness facilitates physiologic load transfer and helps minimize stress shielding. The Trabecular Metal struts generate a friction coefficient that is 76% greater than a sintered bead coating, providing increased initial stability. 5 Exceptional Bone Ingrowth The bone-like physical and mechanical properties of Trabecular Metal contribute to extensive bone infiltration. A transcortical implant animal study demonstrated that new bone rapidly infiltrated the Trabecular Metal. 1,2 Only 8 weeks after surgery, bone had grown into and filled the majority of available pore space. Consequently, fixation
strength developed more rapidly. At 4 weeks, the bone interface shear strength of Trabecular Metal was double that of sintered beads. 1,2 Histologic Micrographs Filling of prepared cortical holes with new bone is comparable with Trabecular Metal implants (top) and without (bottom). 2 Bone filled the majority of the available pore space at 8 weeks. Trabecular Metal has been shown to permit physiologic bone healing. In 24 week animal studies of Trabecular Metal acetabular cups, the density of ingrown bone was comparable to the density of peri-implant trabecular bone. 6 Soft Tissue Attachment The pore size and high volume porosity of Trabecular Metal supports vascularization and rapid, secure soft tissue ingrowth. In the canine model, soft tissue adherence and vascularization occurred quickly with extensive tissue ingrowth 4 to 8 weeks after surgery. Soft tissue attachment strength was five times greater than with sintered bead coatings at 4 and 8 weeks. 7,8
Product Applications Trabecular Metal, a structural biomaterial in many cases, does not require a solid metal substrate and can be fabricated into complex implant shapes. Clinical experience has demonstrated its versatility in diverse bone and soft tissue applications. 9,10 Modular metal prosthetic augments have been used for several decades for treatment of bone deficiency during revision knee arthroplasty. A new acetabular reconstructive technique has been developed with the use of modular metal augments using porous tantalum. These porous metal acetabular augments were developed to be used similarly to structural bone allograft techniques to achieve simultaneously biologic fixation and provide coverage and mechanical support for an uncemented hemispheric acetabular component. Multiple sizes and shapes of these acetabular augments accommodate the various acetabular bony defects encountered in addition to the various sizes of hemispheric acetabular components. Modular acetabular augments were implanted in 16 patients (16 hips) for support of an uncemented hemispheric acetabular component during revision acetabular reconstruction.
Based on the classification of Paprosky, acetabular bone defects were classified as 2A in one hip, 2B in three hips, 2C in one hip, 3A in five hips, and 3B in six hips. Preoperatively, the prosthetic femoral head centers were located at a mean horizontal distance of 18.6 mm (range, -3 46 mm), and a mean vertical distance of 27.6 mm (range, -16 52 mm) from the approximate femoral head center. Postoperatively, the prosthetic femoral head centers were located at a mean horizontal distance of 10.4 mm (range, 1 25 mm), and a mean vertical distance of 7.4 mm (range, -15 25 mm). At final follow-up, no implant had evidence of migration or loosening. At early clinical followup, this modular acetabular augment system seems helpful in acetabular reconstructions that cannot be treated with an uncemented hemispheric cup that would have required other forms of treatment such as structural allografts, acetabular cages, bilobed acetabular components, or custom acetabular components. Longer term follow-up is required to determine whether there are untoward effects of using a modular acetabular reconstructive system. References 1. Bobyn JD, Stackpool G, Toh K-K, et. al. Bone ingrowth characteristics and interface mechanics of a new porous tantalum biomaterial. J Bone Joint Surg. 1999; 81-B:907-914. 2. Bobyn JD, Hacking SA, Chan SP, et. al. Characterization of a new porous tantalum biomaterial for reconstructive orthopaedics. Scientific Exhibit, Proc of AAOS, Anaheim CA, 1999. 3. Black J. Biological performance of tantalum. Clin Materials. 1994;16:167-173. 4. Krygier JJ, Bobyn JD, Poggie RA, et. al. Mechanical characterization of a new porous tantalum biomaterial for orthopaedic reconstruction. Proc SIROT. Sydney, Australia, 1999. 5. Fitzpatrick D, Ahn P, Brown T, et. al. Friction coefficients of porous tantalum and cancellous and cortical bone. Proc 21st Ann Amer Soc Biomechanics. Clemson SC, 1997.
6. Bobyn JD, Toh K-K, Hacking SA, et. al. The tissue response to porous tantalum acetabular cups: A canine model. J Arthroplasty. 1999; 14:347-354. 7. Hacking SA, Bobyn JD, Toh K-K, et. al. Fibrous tissue ingrowth and attachment to porous tantalum. J Biomed Mater Res. 2000; 52:631-638. 8. Bobyn JD, Wilson GJ, MacGregor DC, et. al. Effect of pore size on the peel strength of attachment of fibrous tissue to porous surfaced implants. J Biomed Mater Res. 1982; 16: 571-581. 9. Christie MJ, DeBoer DK, Schwartz HS. Total knee arthroplasty and limb salvage with a custom tantalum femoral component. Inter. Society of Tech in Arthroplasty, Berlin, 2000. 10. O'Keefe T, Cohen RC, Averill RA, et. al. Design principles of a novel monoblock acetabular cup. Proc Australian Orthopaedic Association, Brisbane, Australia, 1999. Some photos courtesy of JD Bobyn, PhD, Jo Miller Orthopaedics Research Laboratory, McGill University, Montreal, Canada. *Manufactured by Implex Corp. +Testing performed in animal models.