Fixation of hydroxyapatite-coated revision implants is improved by the surgical technique of cracking the sclerotic bone rim

Treatment of osteonecrosis of the femoral head by core decompression and implantation of fully functional ex vivo-expanded bone marrow-derived mesenchymal stem cells: a proof-of-concept study

Background

Based on several attributes involved in bone formation, bone marrow-resident mesenchymal stem cells (MSCs) have been employed in the treatment of patients suffering from femoral head osteonecrosis. Due to the low content of MSCs in the bone marrow, ex vivo expansion procedures are utilized to increase the cell number. Customarily, before administration of the resulting expanded cell product MSCs to the patient, its cellular identity is usually evaluated according to a set of “minimal phenotypic” markers, which are not modified by ex vivo processing. However, MSC functional (“reparative”) markers, which are severely impaired along the ex vivo expansion routine, are usually not assessed.

Patients and methods

In this proof-of-concept study, a cohort of five avascular osteonecrosis patients received an instillation of ex vivo-expanded autologous MSCs, manufactured under controlled conditions, with an aim to protect their functional (“reparative”) capacity.

Results and conclusion

Outcomes of this study confirmed the safety and effectiveness of the MSC-based therapy used. After a follow-up period (19–54 months), in all patients, the hip function was significantly improved and pain intensity markedly reduced. As a corollary, no patient required hip arthroplasty.

Do Bone Graft and Cracking of the Sclerotic Cavity Improve Fixation of Titanium and Hydroxyapatite-coated Revision Implants in an Animal Model?

BACKGROUND

We previously introduced a manual surgical technique that makes small perforations (cracks) through the sclerotic bone shell that typically forms during the process of aseptic loosening ("crack" revision technique). Perforating just the shell (without violating the proximal cortex) can maintain overall bone continuity while allowing marrow and vascular elements to access the implant surface. Because many revisions require bone graft to fill defects, we wanted to determine if bone graft could further increase implant fixation beyond what we have experimentally shown with the crack technique alone. Also, because both titanium (Ti6Al4V) and hydroxyapatite (HA) implant surfaces are used in revisions, we also wanted to determine their relative effectiveness in this model.

QUESTIONS/PURPOSES

We hypothesized that both (1) allografted plasma-sprayed Ti6Al4V; and (2) allografted plasma-sprayed HA-coated implants inserted with a crack revision technique have better fixation compared with a noncrack revision technique in each case.

METHODS

Under approval from our Institutional Animal Care and Use Committee, a female canine animal model was used to evaluate the uncemented revision technique (crack, noncrack) using paired contralateral implants while implant surface (Ti6Al4V, HA) was qualitatively compared between the two (unpaired) series. All groups received bone allograft tightly packed around the implant. This revision model includes a cylindrical implant pistoning 500 μm in a 0.75-mm gap, with polyethylene particles, for 8 weeks. This engenders a bone and tissue response representative of the metaphyseal cancellous region of an aseptically loosened component. At 8 weeks, the original implants were revised and followed for an additional 4 weeks. Mechanical fixation was assessed by load, stiffness, and energy to failure when loaded in axial pushout. Histomorphometry was used to determine the amount and location of bone and fibrous tissue in the grafted gap.

RESULTS

The grafted crack revision improved mechanical shear strength, stiffness, and energy to failure (for Ti6Al4V 27- to 69-fold increase and HA twofold increases). The histomorphometric analysis demonstrated primarily fibrous membrane ongrowth and in the gap for the allografted Ti6Al4V noncrack revisions. For allografted HA noncrack revisions, bone ongrowth at the implant surface was observed, but fibrous tissue also was present in the inner gap. Although both Ti6Al4V and HA surfaces showed improved fixation with grafted crack revision, and Ti6Al4V achieved the highest percent gain, HA demonstrated the strongest overall fixation.

CONCLUSIONS

The results of this study suggest that novel osteoconductive or osteoinductive coatings and bone graft substitutes or tissue-engineered constructs may further improve bone-implant fixation with the crack revision technique but require evaluation in a rigorous model such as presented here.

CLINICAL RELEVANCE

This experimental study provides data on which to base clinical trials aimed to improve fixation of revision implants. Given the multifactorial nature of complex human revisions, such a protocoled clinical study is required to determine the clinical applicability of this approach.

Functionally deficient mesenchymal stem cells reside in the bone marrow niche with M2-macrophages and amyloid-β protein adjacent to loose total joint implants

We sought to demonstrate whether there is a difference in the local mesenchymal stem cells (MSC) niche obtained from patients undergoing their first total joint replacement surgery versus those patients undergoing a revision surgery for an failing total joint implant. Bone marrow aspirates collected from patients undergoing revision total joint arthroplasty were observed to be less clonal and the expression of PDGFRα, CD51, ALCAM, endoglin, CXCL12, nestin, and nucleostemin were decreased. Revision MSC were also less able to commit to an osteoblast-lineage or an adipocyte-lineage. Further, in revision MSC, OPG, and IL6 expression were increased. Monocytes, derived from revision whole marrow aspirates, were less capable of differentiating into osteoclasts, the cells implicated in the pathologic degradation of bone. Osteoclasts were also not observed in tissue samples collected adjacent to the implants of revision patients; however, the alternatatively activated M2-macrophage phenotype was observed in parallel with pathologic accumulations of amyloid-β, τ-protien and 3-nitrotyrosine. Despite the limited numbers of patients examined, our data suggest that nucleostemin may be a useful functional marker for MSC while the observation of M2-macrophage infiltration around the implant lays the foundation for future investigation into a novel mechanism that we propose is associated with loose total joint implants.

Coating with a modular bone morphogenetic peptide promotes healing of a bone-implant gap in an ovine model

Despite the potential for growth factor delivery strategies to promote orthopedic implant healing, there is a need for growth factor delivery methods that are controllable and amenable to clinical translation. We have developed a modular bone growth factor, herein termed “modular bone morphogenetic peptide (mBMP)”, which was designed to efficiently bind to the surface of orthopedic implants and also stimulate new bone formation. The purpose of this study was to coat a hydroxyapatite-titanium implant with mBMP and evaluate bone healing across a bone-implant gap in the sheep femoral condyle. The mBMP molecules efficiently bound to a hydroxyapatite-titanium implant and 64% of the initially bound mBMP molecules were released in a sustained manner over 28 days. The results demonstrated that the mBMP-coated implant group had significantly more mineralized bone filling in the implant-bone gap than the control group in C-arm computed tomography (DynaCT) scanning (25% more), histological (35% more) and microradiographic images (50% more). Push-out stiffness of the mBMP group was nearly 40% greater than that of control group whereas peak force did not show a significant difference. The results of this study demonstrated that mBMP coated on a hydroxyapatite-titanium implant stimulates new bone formation and may be useful to improve implant fixation in total joint arthroplasty applications.