3D PSI Arthroplasty

Surgeons and engineers are always looking for new ways to optimize the surgical placement of joint arthroplasty implants. Patient-specific instrumentation (PSI) is being developed through rapid prototyping, and it has already been successfully put into massive clinical application for knee arthroplasty. Although 3D PSI has been utilized in shoulder arthroplasty, it is unclear whether it improves accuracy and results when compared to traditional approaches in either shoulder or knee arthroplasty. PSI has been confined in the hip to the placing of custom-made implants and a small number of surgeons evaluating developing options from various manufacturers. Early results show that using PSI in hip arthroplasty yields in constant, correct implant location, but at a higher cost and with an unknown influence on clinical outcomes.

3D PSI and Joints Arthroplasty

The surgical implant and patient factors all influence the clinical function of hip arthroplasty. Component positioning, for example, is a modifiable risk factor that has been linked to patient dissatisfaction, displacement, wear, impingement, limited range of motion, and leg length inequality. Acetabular loosening occurs following hip arthroplasty in up to 50% of cases. Computer-assisted surgery and robotics have been utilized to improve component positioning precision in hip arthroplasty, but adoption has been slow because of high prices, longer operating times, and other logistical challenges. 3D PSI hip arthroplasty has been reported to be successful.

Patient-specific instrumentation (PSI) has recently been created to aid component location during hip arthroplasty. This method employs CT and MRI imaging techniques to plan surgery in a virtual three-dimensional (3D) environment. The surgeon can then plan implant orientation and position relative to a standard frame of reference and carry out the plan with the use of simple intraoperative patient-specific instructions.

Since its inception in the mid-1980s for the production of products and household machines, rapid prototyping has been employed for a wide range of medical and nonmedical applications. PSI developed from this, and it is now used in a variety of surgical specialties. The first reported usage in orthopedics was for developing guides to aid in the placement of pedicle screws in the spine in 1998, and it has since expanded to include guides for shoulder, hip, knee, and ankle elective and trauma surgery.

PSI is increasingly being utilized commercially in total hip arthroplasty to create custom-made pinning and cutting guides; however, new study has questioned whether PSI enhances implant alignment accuracy or clinical results when compared to traditional instrumentation. PSI for shoulder and ankle arthroplasty has shown encouraging early results in terms of implant placement accuracy; however, commercial applications have yet to be established, and no study has shown improved clinical outcomes in shoulder or ankle arthroplasty.

3D PSI Application in Hip Arthroplasty

In hip arthroplasty, PSI is utilized to increase acetabular and femoral component positioning precision. Cup size, implant medialization, anteversion, and inclination are all goals of acetabular guiding systems. Femoral guiding systems attempt to optimize stem size and location, as well as offset, leg length, and stem version. Internationally, four commercial systems are now available. A number of patents have also been published on devices that have been updated in order to improve on existing designs. In the United States and Europe, the Signature Hip (Zimmer Biomet), OPS (Corin Group), and MyHip (Medacta) are currently available.

To construct the patient-specific model and pattern the guides and implants required, all systems required preoperative imaging with either CT or MRI as well. CT has difficulties in depicting soft tissue and gives well-defined bone anatomy with low degrees of artifact. The commercial systems available use a low-dose radiation regimen that delivers slightly more radiation exposure than conventional radiography, with a scan period of 11 minutes and a direct patient cost of 55$.

All four commercially marketed hip PSI systems provide guidance for acetabular component alignment, but only two include a guide for femoral component orientation. Surgical techniques vary per product, with anterior, posterior, and lateral surgical approaches accessible based on the system.

Although future models may have a guiding component, the depth of acetabular reaming and hence planning for correct medialization of the implant are still determined by hand and professional judgment. If required, pins can be utilized to guide the reamer's position. Before trying implantation into the patient, 3D printed acetabular models are available to aid in understanding the patient's specific anatomy and how the guide/implant should fit.

Constrained and unconstrained insertion guides are divided into two groups, depending on whether the guide just depicts the correct implantation direction or physically assists insertion. Bony and soft-tissue landmarks (such as the transverse acetabular ligament) are used to implant a custom template into the acetabulum, ensuring accurate insertion of the guides. The surgical exposure is critical at this step, just as it is in conventionally guided surgery. Pins or lasers are used to guide the placement of the implants. All require caution when preparing the surgical site, especially when removing soft tissue to avoid removing landmarks required in planning.

Does 3D PSI Improve the Accuracy of Cup Orientation?

Buller et colleagues conducted a dry bone simulation trial with seven surgeons conducting traditional total hip arthroplasty followed by PSI-guided complete hip arthroplasty. The surgical goal was to precisely insert the acetabular implant in 22 degrees of anteversion and 40 degrees of inclination, as planned. Six of the seven acetabular components in the standard instrumentation group were in an unsatisfactory position in terms of inclination and version. Three of the seven acetabular components in the PSI group were misaligned in terms of version, but none were misaligned in terms of inclination.

Shandiz et al performed a postmortem study in which 12 hips were implanted with PSI-guided acetabular components. Preoperative CT scans were utilized to plan the surgery, and postoperative CT scans were performed to assess implant placement and orientation. This demonstrated the ability to position the component precisely utilizing the PSI guide to within 2.5 degrees of the planned position, with a maximum deviation from the planned position of 2.7 degrees.

Schwarzkopf et al used the Bullseye Hip Replacement Instruments PSI for acetabular preparation and cup implantation in a postmortem study of 14 acetabular components. Preoperatively, CT and MRI were used to evaluate the surgical plan and build the PSI. The precision of implant placement was demonstrated using postoperative CT images. All implanted dimensions matched the preoperative surgical planned implant dimensions, and the acetabular cup inclination and anteversion angles were within the desired range.

Small et al examined 18 patients receiving Total hip arthroplasty (THA) with conventional instruments with 18 patients undergoing THA with PSI in a prospective randomized controlled study. CT images were used to compare anticipated and actual results before and after surgery. The findings revealed a statistically significant difference between standard instrumentation and patient-specific instrumentation (PSI) in the version of the acetabular component. The inclination difference was not statistically significant.

Spencer- Gardner et al treated 100 patients with the PSI for cup placement in a prospective study. They used 3D CT planning software to determine the best acetabular inclination and version for each patient before the surgery. Each patient had a posterolateral surgical approach, with the acetabular implants being precisely placed using the PSI laser guidance system. The accuracy of implant placement was determined by comparing the actual position to the preoperative plan using 3D CT. They demonstrated precise placement to within 5 degrees in 54 percent of cases and to within 10 degrees in 91 percent of cases. These results are equivalent to those obtained using robotic and computer-assisted navigation approaches.

Some pitfalls to successful cup implantation have been identified, including mistakes committed during the cup implantation impaction procedure. At this point, extra caution is advised. Furthermore, the presence of osteophytes can complicate the proper placement of the PSI guide and must be considered when carrying out the surgical plan.

Does the Use of 3D PSI Affect the Duration of Surgery?

According to Hananouchi et al, the average surgical time with PSI was 106 minutes, compared to 116 minutes with normal instrumentation. This distinction was insignificant. The surgical guide was used on average for 3.6 minutes in the PSI group.

Spencer-Gardner et al discovered that using PSI increases total operation time by 3 to 5 minutes. This compared to time increases of 8 to 58 minutes in navigated THA. PSI was utilized by Ito et al for femoral component insertion, and the average surgery time was 111 minutes. Small et al showed that the PSI group had a mean surgery time of 95 minutes compared to 88 minutes for the normal instrumentation group; however, this difference was not statistically significant.

Is 3D PSI Useful in Cases with Massive Bone Defects and Abnormal Anatomy?

The use of currently available PSI guidelines for hip arthroplasty is contraindicated in patients with significant deformity and insufficient bone structure or quality. For directing instruments intraoperatively, the dynamic modeling employed with PSI requires proper anatomy and locations of hard attachment. With further advancements in the design of these guides, these systems may be able to be used in patients with more severe deformities.

How Much Does 3D PSI Add to the Operation Costs?

Most surgeons would use commercially supplied PSIs, which add an average of $370 to each surgical case. Hananouchi et al projected initial costs of up to 150,000$ for individual hospital PSI production in 2010, including 15,000$ to 30,000$ for software and 120,000$ for the rapid prototyping machine itself, as well as a cost of materials per case of 50$ to 100$. Although the long-term impact of this guidance on minimizing the need for future medical treatment for early failures of improperly positioned implants is unknown, the potential financial benefits may likely outweigh the additional upfront expenses.

Does the Use of 3D PSI Have Any Other Intraoperative Effects?

Blood Loss

Hananouchi et al found that PSI caused 655 mL of blood loss compared to 683 mL for conventional instrumentation; however, this difference was not statistically significant. Using PSI for the femoral component, Ito et al reported a mean predicted blood loss of 356 mL. Small et al discovered that the PSI group lost 200 mL of blood on average compared to 150 mL in the conventional instrumented group; however, this difference was not statistically significant.

Postoperative Complications

In a study of 100 patients who received PSI for cup placement, Spencer-Gardner et al reported one complication: a fragmented ceramic liner due to inadequate seating, necessitating revision liner exchange. In a study of 10 patients, Ito et al reported no problems or revision operations after using patient-specific instruments for femoral stem implantation. Small et al found no complications in the PSI group and one complication in the conventional instrumentation group in a randomized controlled trial of 18 PSI versus 18 standard instrumentation cases.

How Does 3D PSI Compare with Robotic and Computer-Navigated Systems?

3D CT scans are also used in robotic-assisted hip arthroplasty surgery, allowing for preoperative implant positioning and sizing planning. Robotic-assisted systems are either designed to physically prepare the bone (active) or to prevent the surgeon from reaming beyond the predefined boundaries. Despite the fact that robotic-assisted surgery has been proven to be a reliable surgical method, it has been met with skepticism due to worries about its simplicity of use and price.

CT, fluoroscopy, or imageless navigation techniques are used in computer-navigated hip arthroplasty surgery. CT navigation is relatively unaffected by minimal access techniques, and long-term clinical outcomes have been positive. Cost and radiation exposure issues with CT navigation, as well as worries about accuracy with fluoroscopic and imageless navigation, have inhibited surgeon acceptance.

PSI for hip arthroplasty is still in the early stages of clinical application; while studies establishing implant location accuracy appear promising, longer-term clinical data has yet to be published. PSI also raises the overall expense of the procedure, as well as possibly lengthening the surgery time, depending on the complexity of the case and the surgeon's skill.

Areas Where 3D Printing Playing an Important Role in Surgery

• Preoperative evaluation: an actual life model allows the surgeon to see and feel the disease pathology in all three dimensions. Orthopedic surgeons, joint replacement surgeons, cardiac surgeons, and maxillofacial surgeons will find this particularly valuable. Oncology surgeons will benefit from the technology since it allows them to plan ideal resections and reconstructions.
• Patient-specific (customized) instruments: the technology has advanced significantly in the development of patient-specific instruments. Rapid prototyping can be used to create appropriate jigs and cutting tools after the back-end office has completed the design and simulation. Certain firms are working hard to develop this technology, and several proprietary devices, like as Biomet's Signature and Smith & Nephew's Visionaire, are already accessible internationally.
• Customized implants: individualized implants will be used in the near future instead of one-size-fits-all implants for people and certain disorders. This will not only extend the implant's life and improve its kinematics, but it will also ensure that natural, non-damaged portions are preserved. In the United States, ConforMIS is already available for limited use as patient-specific implants.
• Patient education: in addition to serving as a teaching tool for surgical residents and fellows, 3D-printed models can also be used to educate patients. Patients can have a better understanding of their disease process, planned interventions, and participate in well-informed decision-making. This is especially true for therapies in which the technical details can be daunting for patients and their families.

3D PSI Printing Technologies Available

Fused Deposit Modeling

This is also known as additive manufacturing and is the most frequent technology available to surgeons. A spool of thermoplastic material is put into an extrusion head, which warms the material into a semisolid state. The semisolid thermoplastic or equivalent substance is then extruded by the extruder head. The axial image is converted into a machine-printable language that the machine delivers layer by layer as a reproduction of the axial cuts using specialized software.

Direct Digital Manufacturing

In this situation, the equipment produces the final product directly. The machine prints the material that is suitable for final use, so this printed product is ready to use. Implants constructed of novel materials such as titanium and tantalum, as well as bio-ceramics such as hydroxyapatite and tricalcium phosphate, could be examples in the medical profession. With this technology on hand, patients can receive a tailored product, such as wedges, spacers, prosthetics, or artificial bones for abnormalities. Most prostheses and implants available in the future, according to the technology enthusiast, will be manufactured utilizing this technology.

Polyjet

This technology aids in the creation of highly precise pieces and has the added benefit of being able to blend various materials and colors. These printers, which are similar to inkjet printers used in everyday life, may help build models with over a thousand physical attributes and colors.

Conclusion

In the face of rising disease burden and a bleak economic outlook, additional cost-effective operational procedures to treat end-stage knee osteoarthritis are clearly needed. Unicompartmental knee replacement has been found to be more cost-effective than total knee replacement in suitable patients of all ages, but concerns about revision rates when performed by low-volume, inexperienced surgeons are a barrier to its increased use, which may be related in part to component positioning accuracy.

PSI has been shown to give promising levels of accuracy for Unicompartmental knee replacement in the hands of competent surgeons in an era of growing customized medical treatment. The ultimate test will be whether accuracy can be delivered in the operating room for inexperienced surgeons, as evidenced by improved patient observed outcome metrics and lower revision rates. Because such advantages are not always visible right once, technology like PSI will need to demonstrate other benefits in the meanwhile, such as increased theater efficiency and lower procedure costs, if it is to be broadly implemented.

PSI hip guides have been proven to increase implant positioning accuracy and may have a role to play in complex anatomy reconstruction, particularly in revision surgery. However, it is unknown whether the use of PSI in hip arthroplasty has any long-term functional or survival consequences. More clinical outcome data is needed to persuade the surgeon that the accuracy benefits of PSI outweigh the obstacles of the learning curve and the increased costs.