Bone Quality and Orthopaedic Surgery

Introduction:

Osteoporosis and osteopenia, characterized by impaired bone structure, mass, and strength, have become increasingly common as our population ages. While osteoporosis has many socioeconomic implications, it also carries significant medical consequences. These can include delayed fracture healing and poor outcomes following surgeries as they rely on successful implant osseointegration such as spinal fusion and arthroplasty. This whitepaper will examine how osteoporosis is diagnosed, its impact in orthopaedic procedures, and how additive manufacturing and advanced nanotechnology may provide solutions to these problems.

Diagnosis:

Osteoporosis is diagnosed with either dual X-ray absorptiometry (DXA) or more recently with computer tomography bone mineral density studies. While both studies can be used to determine bone mineral density, neither can provide details on the microstructure of the bone, or differentiate between cortical or cancellous bone loss (Figure 1). Ultimately, this loss of bone mineral density accompanied with a change in the microstructure can have a significant impact on healing following orthopaedic surgery procedures. These changes come with an increased risk of fragility fractures of the hip, wrist, and spine, as well as a decreased ability to heal these fractures and achieve osseointegration in orthopaedic procedures such as arthroplasty and spinal fusion which are highlighted below.

Osteoporosis and Arthroplasty:

Arthroplasty, the gold-standard treatment for end stage osteoarthritis, is routinely performed on older patients who are in turn more likely to have lower bone mineral density. It has been estimated that upwards of 23% of patients undergoing arthroplasty will have osteoporosis while another 45% will have osteopenia at the time of surgery1. Due to their softer bone, osteoporotic and osteopenic patients undergoing arthroplasty are at higher risk of numerous complications including implant subsidence and migration, intraoperative fracture, and periprosthetic fracture2. In an attempt to avoid these complications, proper implant selection and surgical technique play an important role in this patient population.

Historically, the consensus has been to use cementless porous coated total hip implants in younger patients with better bone quality as it is thought that patients without osteoporosis are more likely to achieve successful bony ingrowth and osseointegration3. Conversely, patients with osteoporosis or osteopenia will frequently receive a cemented total hip implant, to allow for earlier fixation and stability as they are less likely to achieve successful osseointegration4. While cementation is routinely performed safely, it does carry significant risk such as “bone cement implantation” syndrome, which is characterized by sudden cardiac arrest secondary to a cement embolus entering into the circulatory system5. As such, many surgeons approach cementation with caution as they search for alternative options to achieve bony stability in these challenging cases.

Osteoporosis and Spinal Fusion:

As our aging population will need to have their joints replaced for arthritis, we are also poised to see a significant increase in other degenerative conditions such as degenerative spondylolisthesis, disc disease, and adult spinal deformity. Spinal fusion surgery is commonly part of the treatment algorithm for these conditions, and similar to arthroplasty is highly dependent on bone quality. As expected, the rates of osteoporosis and osteopenia in patients undergoing spinal fusion are similar to that of those undergoing arthroplasty with roughly 2/3 of patients having some form of impaired bone quality1 .

Osteoporotic and osteopenic patients undergoing spinal fusion procedures are at risk for many significant complications such as implant loosening, specifically pedicle screw pull-out, interbody cage subsidence, and failure to achieve stable posterolateral fusion6-8. These complications do not come as a surprise as successful spinal fusion relies on a bony ingrowth and osseointegration to achieve stable fixation. Patients who suffer these complications frequently have worse outcomes and may ultimately require a revision procedure to fix their underlying condition. In an attempt to address poor bone quality at the time of spinal fusion, surgeons have attempted cement augmentation of pedicle screws and vertebral bodies, but these procedures do not come without risk and are applied sparingly. Just as in arthroplasty, spine surgeons are eagerly searching for solutions for obtaining stable spinal fixation in osteoporotic patients.

Implant Design to Address Osteoporosis:

Addressing the impact of osteoporosis in orthopaedic surgery will require a multifaceted approach. Numerous pharmacological agents such as bisphosphonates and recombinant antibodies are currently being used to address bone mineral density and microstructure. In addition, implant design has emerged as a method to address poor bone quality at the time of surgery. Recent advances in manufacturing techniques such as additive manufacturing, or 3D printing, have allowed for the creation of highly complex lattice structures with improved osseointegration capacity. Implants can now be designed to have an optimized lattice structure (see lattice structure white paper) in areas where bony ingrowth are important to prevent complications such as loosening and subsidence. Additionally, advanced nanotechnology coatings with materials such as hydroxyapatite and even bisphosphonates can be selectively applied to the surface of implants to possibly promote bone formation and osseointegration. Ultimately, these advanced manufacturing techniques may represent the solution to poor bone quality in orthopaedic surgery.