Traditional hip replacement involves the use of a cobalt chrome or titanium femoral stem with a cobalt chrome femoral head that articulates with a polyethelene liner in an acetabular cup. While polyethelene remains the bearing material of choice in joint replacement procedures, traditional poly has been superceded by its enhanced form. Through radiation processes, these enhanced polyethelenes undergo a structural change that more tightly bonds their molecules. The resultant material reportedly exhibits improved abrasion and wear characteristics over traditional UHMWPE, thereby extending its longevity. Although some concerns exist about the long term wear properties of some of these highly cross-linked polyethelenes, they are used in a majority of hip cases and an increasing number of knee cases. Other companies have taken enhanced poly one-step further by stabilizing it with vitamin E.
Most key players in joint replacement also market alternate bearing constructs that center on metals and ceramics. However, metal-on-metal constructs have fallen from 30% of hips sold in 2009 to less than 5% in 2011 due to mixed clinical results, negative press and poor public perception. Ceramic-on-ceramic implants have faced a decline as well.
Recently, several companies have introduced mobile bearing hips to address dislocation issues without metal-on-metal bearings. These systems feature a dual mobility acetabular system with advanced bearing technology, which reportedly provides significant wear reduction versus conventional polyethelene.
While the use of alternate bearings serves to address a younger, more active patient, so too do larger diameter head systems, which allow for increased range of motion and lower risk of postoperative dislocation. Today, larger diameter heads remain part of companies’ full line of hip replacement products.
Finally, many companies have introduced conservative joint replacement technologies like bone-preserving hips, mini stems, and short stems.
As in hip replacement, enhanced polyethelene plays a role in reducing wear in knee replacements. So too do less-constrained mobile bearing knee designs, although only two companies have secured FDA approval to market such devices in the US.
Ceramics also play a role in knee systems, as exemplified by the use of oxizided zirconium material, a proprietary transformed metal alloy with a ceramic bearing surface.
Materials aside, a key trend in knee replacement has been the use of customized instrumentation. By taking preoperative CT or MRI scans, this technology applies preoperative planning software and rapid manufacturing technology to create a template and cutting a jig that accurately fit the unique anatomy of each individual patient. One option is to offer custom implants and instrumentation. Another is offering robotics to assist surgeons in conservative knee procedures and most recently, in hip procedures.
Specializing in Orthopedic Implant Engineering of the Hip and Knee