Perspective on Hydroxyapatite-Coated Collars for Bone-Bridging
Commentary on an article by Melanie Jean Coathup, BSc(Hon), PhD, et al.: “Long-Term Survival of Cemented Distal Femoral Endoprostheses with a Hydroxyapatite-Coated Collar. A Histological Study and a Radiographic Follow-up”
Douglas J. McDonald, MD

Commentary

Osteointegration of metallic implants, in one form or another, has been and remains an important goal of orthopaedic surgeons. Providing a durable, long-lasting reconstruction of diseased joints and skeletal defects with minimal risk of aseptic loosening is a key component in providing pain-free function for our patients.

Successful osteointegration of a metallic implant requires a porous surface on at least part of the implant. To be effective, the surface must be biocompatible, well fixed to the metal substrate, and have a pore structure that essentially allows the bone to “grow” into the surface. Numerous materials and techniques have been used to create this porous surface with varying degrees of biological, biomechanical, and clinical success. Plasma-sprayed hydroxyapatite coating is one method of creating a biocompatible porous surface. The overall results of hydroxyapatite-coated implants have been favorable. There are numerous reported series in which utilization of these implants for cementless fixation of total hip and total knee prostheses provided clinical results comparable with those associated with other forms of fixation.

In this paper, Coathup et al. reported the results of a cemented distal femoral endoprosthesis that has a hydroxyapatite-coated collar at the junction of the stem and the body portion of the prosthesis. While the purpose of the hydroxyapatite coating is still to provide bone ingrowth, conceptually this is an entirely different biological and biomechanical use of the material. The goal here is not to provide primary fixation of the component since this is done by cementing the stem. The goal is to provide for extracortical bone-bridging, a concept that theoretically should be beneficial but is of uncertain value clinically.

It is clear that the authors and their institution have tremendous experience with endoprosthetic reconstruction of skeletal defects after tumor resection. This paper will join their already large body of work and should help us to better understand the results of these procedures and implants. Coathup et al. evaluated sixty-one patients who had undergone reconstruction with this hydroxyapatite-coated-collared prosthesis and primarily analyzed the clinical and radiographic results. In addition, they had the opportunity to histologically analyze four retrieved specimens obtained after amputation due to local tumor recurrence. It is well known, and the authors have stated, that seeing bone growing around the shoulder of an implant radiographically does not necessarily mean that the bone has grown into the implant. In this paper the authors showed us, at least in the four specimens that they analyzed histologically, that bone will grow into the hydroxyapatite coating and therefore achieve some degree of osteointegration. The authors acknowledge that one cannot simply assume that all implants that show radiographic bridging will also have histological bridging, but it is encouraging to note that it can occur.

The radiographic analysis of these implants is also quite interesting. Since hydroxyapatite is primarily a biocompatible scaffold, there is nothing to suggest that it will actually “induce” bone to grow over the shoulder of the implant. This phenomenon can occur over the shoulder of any implant. Yet it is interesting that 30% of the implants in this series showed no extracortical bone growth into the collar. An additional 38% of the patients showed minimal bone ingrowth (in two zones or fewer). While the authors stated that 70% of the patients demonstrated some radiographic osteointegration of the collar, it could equally be concluded that almost 70% did not show any “effective” integration.

It is unknown, of course, how much osteointegration is necessary to achieve the goal of extracortical bone-bridging. In fact, as mentioned earlier, the clinical value of extracortical bone-bridging is itself questionable. It is certainly not necessary for successful long-term fixation of a cemented implant, and the authors have even provided us additional evidence to support this. Their large experience gave them the opportunity to compare this series of distal femoral endoprostheses with a hydroxyapatite-coated collar with another series of 493 of the same prostheses without the collar. Overall, the rate of aseptic loosening was quite low in both groups, and the authors found only a statistically insignificant trend for an increase in survivorship of implants with osteointegration of the collar.

Ultimately, it comes down to what we are trying to achieve with extracortical porous collars, hydroxyapatite-coated or otherwise. It has been suggested that the real benefit of extracortical bone-bridging may be to create a so-called biological purse string, effectively sealing off the bone-stem interface from particulate debris generated at the joint level1. If that is the goal, fibrous-tissue ingrowth alone can do this and is a lot easier to accomplish than bone ingrowth. And while fibrous-tissue ingrowth provides essentially nothing in terms of improved mechanical fixation, if it decreases particle-induced osteolysis it may have the secondary effect of providing lower rates of long-term aseptic loosening.

If the goal is to truly provide osteointegration for the purpose of better mechanical fixation, none of the methods, including hydroxyapatite-coated collars, seem to do this in a predictable manner. Clearly, a well-done cemented stem in the right patient can last a long time. Improvements in stem design and stem options have resulted in effective noncemented fixation as well. It may be that elaborate collars for the purpose of biologic bone fixation are simply not necessary.

In summary, I commend the authors on their excellent paper. Their experience with these implants generates valuable information that the rest of us can utilize to provide the best results for our patients.

Footnotes

  • * The author did not receive payments or services, either directly or indirectly (i.e., via his institution), from a third party in support of any aspect of this work. He, or his institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. The author has not had any other relationships, or engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.

Reference

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