Using Additive Manufacturing to Address Critical Segmental Bone Loss

Introduction:

Many fractures and other bone injuries can be successfully treated in such a way that the bone fully heals. However, some injuries such as motorcycle accidents or pedestrian injuries or chronic issues such as infected bone can result in bone loss that generates a “segmental” gap in the shaft of the bone. The human body is amazing and can repair small gaps, but a “critical segmental bone loss” is one that is so large that it will not heal spontaneously.1; in healthy adults, this is usually about >2 cm in length or > 50% of circumferential bone loss2. If untreated, a critical bone defect will lead to a non-union (bone not healing in continuity) with resultant loss of function of the extremity or even amputation. Management of these devastating injuries poses a significant clinical and socioeconomic burden as these patients will frequently require multiple procedures and prolonged hospitalization with no guarantee of successful healing.

Current Treatment Strategies:

Numerous reconstruction procedures to address critical bone loss exist, all of which aim to bridge the bone defect and allow osteogenic healing. Historically, surgeons have relied on bone grafting from autogenic (the patient) or allogenic (cadaveric) sources, multi-stage surgeries (i.e. Masquelet technique), distraction osteogenesis with an external frame (i.e. Taylor Spatial Frame), or even amputation.

Autologous Cancellous Bone Graft:

Autologous bone graft is commonly used to reconstruct bone defects of less than 5 cm. This procedure involves harvesting the patient’s own bone from the iliac crest (pelvis) and using it to fill the void in conjunction with hardware such as a plate or intramedullary fixation as the bone graft has no inherent structural integrity. As the bone graft is enriched with the patient’s own bone forming cells, this carries increased healing potential. This procedure requires an additional surgical incision and has been associated with significant donor site pain. Due to the limited amount of bone that can be harvested, this technique cannot be utilized for large segmental bone defects such as from tumor research or significant trauma.

Allogenic Bone Graft:

Allogenic bone graft is used in a manner similar to that of autogenic bone graft, however as it comes from a cadaveric source and is typically decellularized, it does not carry significant healing potential, but rather serves as a biological scaffold for the patient’s osteocytes to bridge a gap. Similar to autologous bone graft, allogenic bone graft carries no structural integrity and must be used with other hardware.

Masquelet Procedure:

The Masquelet procedure is an induced membrane technique that allows surgeons to treat segmental bone loss of >5cm with autologous bone graft. In short, this staged procedure involves using polymethyl methacrylate (PMMA) cement to fill the bone done defect and allowing the body to create a membrane around the bone defect. Six to eight weeks later the PMMA cement is removed and the bone void and surrounding membrane can be filled with bone graft. While this can work well for large defects of >5cm, it does carry significant cost and patient dissatisfaction as it requires bone harvesting from the patient and requires at least two stages. Again, there is no structural support.

Distraction Osteogenesis:

Distraction osteogenesis addresses segmental bone defects by transporting a segment of bone in small increments over a long period of time. This is usually done through an intramedullary implant or an external fixator frame that is used to move the bone at a distance of <1 mm/day. This procedure is often lengthy can take 10-12 months to repair a defect of 10 cm.

3D Printed Scaffolds:

Recent advancements in additive manufacturing have allowed for the creation of 3D printed titanium scaffolds that can be used to address critical bone defects. This technique potentially has many benefits over the current standard of care and how surgeons manage these injuries.

In the case of a critical segmental bone loss, advanced modeling software can be used to create a patient specific implant that corresponds to the area of bone loss. With advanced CAD techniques, this implant can be designed to have the same size and shape of the patient’s native bone as modeled from their healthy contralateral extremity. Software can then be used to integrate complex lattice structures into to the implant which can then be printed out in a host of metals including titanium. Other opportunities include the development of standardized augments that may allow patient-matching to established reference designs rather than patient-specific requirements. Additionally, these lattice structures can be designed to match the Young’s modulus of the surrounding bone then minimizing complications such as stress shielding. This implant is then subsequently implanted into the bone defect and can either be further augmented with bone graft and bone graft substitutes, or potentially left as is depending on the nature of the injury.

As previously discussed in our whitepaper, additive manufacturing techniques can allow us to create complex lattice structures with osteoconductive properties that could not previously be made with conventional manufacturing techniques. Additionally, additive manufacturing can allow for the development of patient matched custom implants.