Musculoskeletal 3D Printing

Fig. 8.1
3D model of the spine of a patient with ankylosing spondylitis showing ossification of the anterior and posterior longitudinal ligaments and ligamentum flavum. This can be used for teaching and practicing spinal interventions

3D printing has been increasingly used to preoperatively characterize complex anatomy. Realistic models aid in understanding complex spatial relationships resulting in accurate presurgical planning as precise preoperative measurements are obtained (Fig. 8.2). Thereby, intraoperative time can be significantly reduced. Reduced intraoperative time leads to better utilization of resources and is advantageous as longer operative times have been linked to worse outcomes. Simulated rehearsal of complex surgical steps on 3D models may decrease intraoperative complications. Fixation hardware can be positioned over the patient’s 3D model and pre-contoured to ensure an optimal intraoperative fit. Current applications of 3D printing in presurgical planning include presurgical planning for lesion resection, joint repair and replacement, surgical correction of congenital musculoskeletal deformities, and surgical management of osseous trauma, and are discussed as follows.


Fig. 8.2
20-year-old female with acetabular dysplasia . (a) Radiograph and (b) CT demonstrate shallow acetabulum with uncovering of the femoral heads. (c, d) 3D model provides a much greater impression of the femoral head and acetabular surface

3D printing has proven beneficial in resection of large osteochondroma of the scapula where determining the precise relationship between the mass and the serratus anterior helped avoid potential postoperative scapular wing complication (Tam et al. 2012). In a more complex case, 3D printing was used to successfully carry out en bloc resection of cervical spine primary bone tumors (Xiao et al. 2016). Other degrees of complexity have been described (Kang et al. 2015; Wong et al. 2007a, b; Ma et al. 2016). Utilizing surface characteristics and spatial adjacency of landmarks, 3D bone models reconstructed from CT images have been used to localize and identify critical patient-specific anatomic landmarks (Subburaj et al. 2009). This adds invaluable anatomic information in preoperative planning when identifying important adjacent structures to avoid, such as nerves and vessels.

The intraosseous extent of tumor infiltration needs to be accurately evaluated preoperatively to ensure adequate clearance margin while minimizing the amount of resected bone (Fig. 8.3). This is of paramount importance in patients in whom joint sparing resections are planned. 3D modeling has been used in craniofacial fibrous dysplasia to direct the extent of bone shaving resection while optimizing cosmetic symmetric facial contouring with accurate surgical reduction and shortening of operative time with the use of 3D models (Kang et al. 2015).


Fig. 8.3
40-year-old man with sacral chordoma . (a) CT and (b) MRI of the pelvis show a large sacral mass with sacral destruction. (c, d) 3D model better delineates the margins and extent of osseous involvement which is crucial for presurgical planning and helps in patient education

Functionally successful surgeries involving extremity and pelvic resections are dependent on utilizing custom prostheses to provide near anatomic restoration of function. After obtaining adequate oncologic clearance, further resection needs to be fashioned, so as to snugly accommodate the prosthesis (Wong et al. 2007a, b). An accurate fit of a custom prosthesis depends on precise measurements obtained during presurgical planning so that an apt patient-specific customized prosthesis can be designed. In such patients, two 3D models may need to be printed, one to plan for tumor resection and the other to help guide custom prosthesis planning (Kang et al. 2015).

3D models can be used to select optimum operative hardware, for example, in selection of optimal fixation screw based on a predetermined entry point and expected drill vector during pelvic surgery (Peters et al. 2002). 3D modeling can help design patient-specific instruments (PSI) for intraoperative guidance. Such PSI simplify complex surgical procedures, help making smaller incision size, improve precision of resection, and decrease intraoperative blood loss and overall operating time (Wong et al. 2007a, b). Designing of PSI requires postprocessing extrapolations based on dimensional and geometric specifications which can be further pre-fitted on the preoperative 3D models for further refinement to perfectly align with patient anatomy. They are subsequently used intraoperatively to deliver bone cuts in the target planes resulting in improved accuracy during complex resections (Figs. 8.4 and 8.5). PSI are especially important in pelvic resections where there is limited working space, complex geometry, and decreased intraoperative visibility (Cartiaux et al. 2014). PSI have been used in resection of pediatric proximal tibial sarcomas (Bellanova et al. 2013), chondrosarcoma of the superior pubic ramus (Blakeney et al. 2014), and distal femoral osteosarcoma (Ma et al. 2016) among many others.


Fig. 8.4
Large soft tissue sarcoma (blue arrows) with iliac bone destruction. 3D model better shows bone involvement and relationship to iliac vessels (yellow arrow) for presurgical planning


Fig. 8.5
(a) Patient-specific cutting guide and (b) iliac prosthesis for pelvic reconstruction in the same patient from Fig. 8.4, designed using mirroring techniques. (c) The autoclavable guide printed using ULTEM can be placed intraoperatively for precise surgical incision. (d) 3D models show pelvis after resection. (e) Hemipelvis after implant placement

There is an increasing demand to optimize patient-specific hardware of total joint arthroplasty. It is expected that best quality of life can be achieved by personalized medicine, by fashioning hardware specific to a patient, as against a commercially mass-manufactured prosthesis, due to minor anatomic variations between patients. Patient-specific MRI or CT data define the mechanical and anatomic axes across the joint in planning for an arthroplasty. This data is used to generate the 3D model to accurately plan the size and position of the implant and to fashion a custom implant. In the arena of total joint replacements, 3D printing has led to fashioning custom total joints, intraoperative computer-assisted navigation and surgical guide systems, and PSI which encompasses pinning guides and cutting jigs (Jun 2010; Krishnan et al. 2012). In total knee arthroplasty, tibial and femoral models help create patient-specific cutting guides for preparation of the bone. These custom-made jigs attach to the underlying bone and have slits in their structure which allow cutting through them. Pinning guides help guide accurate pin placement to secure the jigs (Fig. 8.6).


Fig. 8.6
Patient-specific femoral cutting guide for optimal positioning for knee arthroplasty

3D models are especially useful in complex cases of arthroplasty where significant degenerative changes or large areas of bone loss result in complex anatomy. Such altered anatomy requires significant presurgical planning to determine resection level and angle of resection to ensure adequate postsurgical alignment (Schwartz et al. 2015). Minns et al. report positive surgical outcome with the help of 3D models in a patient with rheumatoid arthritis and status post Benjamin’s double osteotomy that had resulted in an unstable varus deformity of the knee and marked deformity of the medial tibial plateau (Minns et al. 2003). 3D models help to predict feasibility, establish optimal surgical strategy, and select the appropriate implant type, size, and position in technically challenging total hip replacements in patients with severe acetabular deficiencies that require structural bone grafting and custom prosthetics and in cases of ankylosis (Won et al. 2013).

Revision total joint arthroplasties are technically challenging due to altered anatomy, decreased available bone stock, and difficulty in achieving stable fixation (Makinen et al. 2016). 3D models aid in presurgical planning of these complex surgeries and assist in the selection of optimum hardware or creation of patient-specific hardware (Fig. 8.7). Cage reconstruction has been utilized to gain rigid fixation in the host bone and bone graft in revision hip arthroplasties. 3D models define the available bone stock, pre-fit the cage construct, and guide surgical technique which decrease the risk of mechanical failure following revision surgery (Li et al. 2013).


Fig. 8.7
50-year-old female with prior pelvic resection and right hip reconstruction. (a) Radiograph, (b) CT coronal image, and (c) CT volume rendering show displacement of the acetabular cup and hardware loosening with associated fractures of the posterior column and the inferior pubic ramus. (d) 3D printed model helped to better analyze the complex spatial orientation of the various components for optimal revision surgery. (e) The model also helped selection of optimal hardware prior to the procedure. This resulted in significant reduction in intraoperative time. (f) Postoperative radiograph shows revised hip arthroplasty with acetabular reconstruction

This technology has been applied to other joints such as rheumatoid arthritis cervical spine fixation and Charcot neuroarthropathy with encouraging results. Cervical spine fixation in rheumatoid arthritis is challenging due to severe deformity, erosive changes, poor bone quality, and aberrant vertebral vasculature. Full-scale 3D models of the cervical spine provide patient-specific stereoscopic mapping of complex anatomy allowing for preoperative fitting of the plate-rod construct for occipitocervical fixation. This optimizes patient alignment and defines parameters for pedicle screw trajectory and point of entry prior to surgery. Such detailed preoperative planning has been shown to decrease postoperative complications such as dysphagia (Mizutani et al. 2008). In Charcot arthropathy, 3D models allow rehearsal of incision site selection, selection of most appropriate instrumentation, determination of feasibility of osteotomy and joint resection levels, and pre-fitting and placement of internal and external fixation devices (Giovinco et al. 2012).

Fabrication of 3D models can allow assessment of joint biomechanics prior to repair. 3D printing can be utilized to quantify the bone loss of the osseous Bankart and Hill-Sachs lesion in a patient with recurrent anterior shoulder instability. Further, 3D model helps determine the degree of abduction and external rotation at which the Hill-Sachs lesion engages. This helps guide appropriate surgical treatment, including the number of suture anchors required for the remplissage procedure and the number of anchors that would fit within the Hill-Sachs (Sheth et al. 2015).

Surgical correction of severe scoliosis is challenging due to loss of traditional anatomic landmarks, risk of major neurologic damage, risk of vascular injury, and unexpected malformations that are often only discovered during surgery, subsequently resulting in prolonged operative time, higher rate of screw misplacement, and increased risk of additional complications. In patients with congenital scoliosis, 3D computed reconstructions have been shown to be more helpful than plain radiographs in identifying posterior vertebral anomalies associated with hemivertebrae (Hedequist and Emans 2003). 3D printing can be more beneficial than 3D computed reconstructions as they allow for comprehensive presurgical evaluation and eliminate risk of encountering unexpected malformations intraoperatively. Further, they provide tactile feedback that can be used in direct rehearsal to refine surgical approach (Mao et al. 2010). The use of 3D printing technology with intraoperative fluoroscopy reduces the risk of transpedicular screw misplacement in patients with scoliosis and its subsequent complications (Wu et al. 2011). These basic principles can be beneficially applied to other congenital pediatric musculoskeletal disorders such as pediatric hip deformities, Blount’s disease, posttraumatic physeal bars, and subtalar coalitions (Starosolski et al. 2013).

3D models have proven important for complex fracture management, improved fracture characterization, more precise anatomic measurements, reduced surgical time (related to pre-contoured fixation hardware, 3D printed patient-specific surgical template guides, pre-planned trajectory, pre-planned type and length of fixation screws), decreased anesthetic dosage requirement, and reduction in intraoperative blood loss and fluoroscopy time (Bagaria et al. 2011; Bizzotto et al. 2015; Wu et al. 2015; Brown et al. 2003). Examples include distal radial fractures, where 3D models provide better appreciation of articular surface gaps ≥2 mm and enable preoperative selection of appropriate fixation hardware (Bizzotto et al. 2016a, b). Similarly in radial head fractures, 3D models increase sensitivity in the diagnosis of fracture line separation of the head from the radial neck, radial neck comminution, articular surface involvement, articular fracture gap greater than 2 mm step-off, impacted fracture fragments, presence of greater than three articular fragments, and presence of articular fracture fragments too small to repair (Guitton et al. 2014). This results in improved consensus in fracture classification and decreased variability in surgical treatment. In chronic fracture deformities like cubitus varus, 3D models help in precise preoperative measurements for the proper location of the osteotomy, amount of wedging required, and the tilting plane of the osteotomy cut, leading to positive results in the restoration of anatomic alignment, functional postoperative outcome, and cosmetic appearance (Mahaisavariya et al. 2006).

Classification and surgical management of complex pelvic and acetabular fractures based on two-dimensional CT images is notoriously difficult. 3D models decrease the degree of interobserver variability in fracture classification and allow for customization of proposed fixation hardware (Hurson et al. 2007; Zeng et al. 2016). For example, in type C pelvic fractures, use of 3D models decreased length of hospital stay and morbidity and accelerated recovery (Li et al. 2015). Pre-contouring of the fracture fixation plates on the mirrored healthy pelvis eliminates the need for intraoperative contouring while treating both-column acetabular fractures, thereby reducing intraoperative time (Upex et al. 2017). 3D printed pelvic osseous models can be overlaid with vascular information obtained from CT angiography. Printed pelvic arteries and veins can be layered in anatomic relation to the fracture fragments to help optimize presurgical planning (Fig. 8.8).


Fig. 8.8
3D model of complex acetabular fracture (blue arrows) involving anterior and posterior column and inferior pubic ramus. Fusion with CT angiography dataset enabled visualization of the relationship of the fracture with iliac vessels (yellow arrows)

3D printing with biocompatible materials can be utilized internally as surgical hardware, external fixation and assistive devices, or as therapy-impregnated implants. Custom fabrication of PSI for routine joint total arthroplasty can improve alignment and reduce intraoperative time (Renson et al. 2014), although a few investigators report no significant improvement or added benefit in routine cases (Voleti et al. 2014; Sassoon et al. 2014). PSI is extremely useful in patients needing nonstandard joint replacement, joint replacement in unconventional anatomy, customized fixation hardware after surgical resection, and patients with significant loss of bone stock after tumor resection. Customized hardware in such patients optimizes fixation biomechanics, thereby increasing stability and decreasing postoperative complications like hardware failure, implant collapse, and subsequent fracture risk. In total hip arthroplasty, for cases of extreme femoral medullary canal narrowing or abnormal anatomic axis of the femoral diaphysis, utilizing 3D printing for fabrication of the custom prosthetic femoral component and the receiving native femoral bone decreases risk of failed prosthetic insertion and intraoperative periprosthetic fracture (Faur et al. 2013). Newer patient-specific ceramic molds allow orthopedic implants to be casted out of a high-resistance cobalt-chrome alloy with built-in submillimeter integral bone ingrowth surface macro-textures which improve bone ingrowth fixation (Curodeau et al. 2000).

3D printed implants made of titanium alloy powder and porous implants have been used to reconstruct multilevel cervical spine in the setting of Ewing’s sarcoma and metastatic papillary carcinoma with good success (Xu et al. 2016; Li et al. 2017). Customized acetabular implant specific to the patient and defect is termed a “triflange” which facilitates precise restoration of acetabular anatomy and hip biomechanics in patients with complex multiple revisions with poor bone stock and pelvis discontinuity (Wyatt 2015). Pelvic implants have been used for complex fractures (Mai et al. 2017) and following complex tumor resections (Wong et al. 2015).

In extremities, customized implants can be used in case of extensive resection or periarticular involvement. Suitable 3D printed endoprotheses can be created using patient-specific mirror image CT data from the normal contralateral extremity (Pruksakorn et al. 2014). Hollow 3D printed calcaneal prosthesis made of titanium has been used after a total calcanectomy for calcaneal chondrosarcoma allowing intraoperative reattachment of the Achilles tendon and the plantar fascia helping patient to become fully weight bearing and mobile in 5 months (Imanishi and Choong 2015). Scaphoid and lunate fractures are the two most common carpal bones affected by avascular necrosis due to limited vascularity (Freedman et al. 2001). The possibility of 3D printed scaphoid and lunate bones with photocurable polymer would allow for implantation of the 3D printed carpal bones with suitable geometry, mechanical properties, and cytocompatibility for in vivo use (Gittard et al. 2009), avoiding other more invasive and extensive surgical options.

3D printing can address current challenges of external prosthetic development including high costs and limited availability. Pediatric prosthetic needs are complex owing to their small size, low weight requirement, constant need for size changes, and subsequent higher cost. In pediatric transradial amputees, 3D printed robotic prosthetics are lightweight and allow individual thumb movement with the ability to grasp objects with all five fingers (Gretsch et al. 2016). A major advantage the ease in scalability of the hand and socket model, allowing uncomplicated printing of new devices, as the patient ages (Fig. 8.9). Such prosthesis can be also be designed and printed remotely where detailed measurements can be extracted from photographs (Zuniga et al. 2015).
Nov 14, 2017 | Posted by in NEUROSURGERY | Comments Off on Musculoskeletal 3D Printing
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