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Patient-Specific Templates for Total Knee Arthroplasty
Mahmoud A. Hafez 1✉ Email
Hosamuddin Hamza 1
1The Orthopaedic Department October 6 UniversityCairoEgypt
Abstract
Advances in computer-assisted techniques, such as patient-specific templates (PSTs), have revolutionized total knee arthroplasty (TKA). Available clinically for more than a decade, PSTs have a TKA success rate comparable to that of traditional knee replacement and that with navigation. With its accurate preoperative calculations, PST allows precise, atraumatic bone cutting and optimal mechanical axis and limb alignment. A surgical procedure may be planned, or the plan supervised, by the surgeon. In addition, the surgery itself can be simulated a priori to confirm the best fit of the template and determine the accurate amount and shape of the subsequent bone cutting. Manufacturing the templates has been made easier with the availability of three-dimensional printers and printing materials. This chapter outlines the history of the development, as well as the clinical and technical setups, of PSTs for use in TKA.
Keywords
Total knee arthroplasty
Patient-specific templates
Hospital-based system
Bilateral simultaneous TKA
4.1.
Introduction
The incidence of knee osteoarthritis continues to increase in our aging populations, mandating the continuing development of surgical techniques using cutting-edge technology to improve total knee arthroplasty (TKA) methods. The aim of patient-specific surgery for TKA is to restore knee function by replacing the deformed knee joint to its original, healthy condition [1]. Widespread application of TKA is still limited because of its technical difficulty, high cost, and the claim that it is useful only in straightforward cases—not for those with severe intra-articular or extra-articular deformities [2]. Another limitation of the current TKA technique is that the knee implants currently available are designed based on Caucasian knees. Recent studies have identified anthropometric differences between Asian/Arab populations and those of Caucasians, which could cause component mismatch and subsequent failure [3].
Patient-specific instruments, also known as patient-specific templates (PSTs), have been introduced in orthopedics as a minimally invasive surgical tool that allows precise execution of preoperative planning, which results in better function and survival rate, shorter operative time, shorter hospitalization and recovery time, and lower complication rate and cost. The PST is therefore considered an alternative to conventional techniques, robotics, and navigation for TKA surgery [4]. It aims to overcome the technical limitations of robotics and navigation while eliminating the drawbacks of the conventional technique. PST for TKA is a custom-made, image-based surgical tool that allows preoperative planning and simulation using computer software, followed by production of the cutting templates according to the surface anatomy of the individual’s bony structures.
Compared with imageless navigation and robotic systems, it has the advantage of preoperative planning, with the surgeon becoming familiar with the given bony structures prior to surgery [5]. The PST system is also considered a simpler, more user-friendly, more affordable tool than surgery that depends on navigation and robotics. It does not require an intraoperative setup or a large operating theater. It has a short operative time because some of the steps usually performed intraoperatively are now done preoperatively. It has a short learning curve and is easily accepted by surgeons, nurses, and patients as a simple modality of computer-assisted orthopedic surgery [6]. Its disadvantage is that it lacks a tool that navigational and conventional techniques have that would provide intraoperative verification of errors, allowing the surgeon to correct the error immediately [7].
4.2.
Technical Setup
The PST requires a rapid prototyping machine—i.e., three-dimensional (3D) printer—for its production via additive manufacturing. 3D printers use liquid, powder, sheets, or filaments to form complex models with predesigned dimensions and structures. The printers are also used to create physical anatomical models reconstructed from computed tomographic (CT) images, such as fractured or deformed bony models for educational purposes or for provisional practice prior to its use in the clinical setting [8]. The use of 3D printing in orthopedics is not limited to TKA. It is also useful for spinal pedicle screw insertion, other joint arthroplasties, and osteotomy.
Currently, the femoral and tibial cutting guides are fabricated with selective laser sintering (SLS) 3D printers using nylon as the material for the cutting guides. This production cycle takes place outside the hospital (being outsourced to other facilities) to avoid the cost of purchasing an expensive machine. The cycle includes writing the patient’s initials and code number, whether it is the right or left knee to be operated on, and the surgeon’s name on the cutting guides [9]. The use of nylon as the material of choice for producing the cutting guides is based on its being autoclavable, hard enough to withstand the cutting forces without flaking at the operating site, and reproducible when used for finely detailed objects. The surgical staff is given information preoperatively regarding the size and type of knee implant and the instruments to be used. Neither traditional instruments nor intramedullary rods are required for the procedure [10].
The concept of PST was first utilized by industrial companies to produce pin locators. The planning is based on either CT or magnetic resonance imaging (MRI) images. The disadvantage of using MRI is that it sometimes gives unpredictable results for obese, unfit, or claustrophobic patients or those with a pacemaker. The only known drawback for CT is its associated radiation exposure [8]. In addition to MRI or CT, long-leg topography, extending from the hip to the ankle in anteroposterior and lateral projections, is required. This long-leg film could be useful for measuring changes in leg length preoperatively and postoperatively [11].
The first author applies an ‟open-platform” technique, wherein a PST could be used for any type or size of knee implant, unlike commercially available implants. MRI systems have an average waiting time of 6 weeks from acquiring the MRI scan to production of the PST and its delivery to the hospital. This long interval carries the risk of anatomical changes in cartilage due to daily activities or as a result of abnormal loading during this waiting time, which may cause misaligned templates and subsequent failure. Failure can also result from inaccurate bone segmentation, which is more likely to occur when using MRI than CT images [4].
The most critical step of TKA is intraoperative positioning of the templates because malpositioning could lead to error in the cutting, with subsequent malalignment of the lower limb. A previous laboratory study tested the accuracy and reliability of PSTs on cadaveric and plastic knees. Its analysis of postoperative CT scans showed that the mean errors for alignment and bone cutting were within 1.7° and 0.8 mm, respectively [12], which is higher than those reported for conventional and navigational techniques (3.0°) in the clinical setting. Another study included five independent observers, who examined the postoperative results (bone cutting, alignment of the prosthesis) of patient-specific TKA performed on plastic knees [4]. The observers were given a navigational tool for measuring the position. Even “naive” users were able to produce acceptable sizing errors (0.32–1.0 mm) and alignment errors (0.67°–2.5°). That is, positioning the PST was reliable, and there were no significant intraobserver/interobserver variations regarding alignment or the levels of bone cutting of the femur and tibia. These studies were useful for evaluating the reliability of the TKA with PSTs. It is not the case clinically, however, as kinematics are more important than mechanical alignment [13].
4.3.
Clinical Setup
During TKA, the knee joint is exposed via the medial parapatellar approach. The femoral template is positioned in a best-fit fashion—that is, by locating probes at the bottom surface of the template that are touching the distal femur at the center and at median and lateral points, this position should be the unique, single point at which the template could be fixed by pins onto the distal femur with an angle or wing to verify the amount of bone to be removed from the distal and frontal sides [14]. Bone is then cut with saw blades of known thickness, inserted through a slit for the distal cut and then for the anterior, posterior, and chamfer cuts. The tibial template is positioned over the proximal tibia at the frontal and medial surfaces and the tibial plateau, as close as possible to the patellar tendon insertion, which requires good clearance of the surrounding soft tissues. This position could be verified with surface matching (i.e., a single, unique matching position) that is secure enough to avoid dislodgement during bone cutting [4]. The template is fixed by the fixation pins at the front and top surfaces with an angle or wing to verify the amount of bone to be removed. Bone cutting is done through the cutting slits, with the cut bone removed from the medial and lateral sides. The templates could be modified by adding lug holes [15]. Stem and keel preparation is done through the corresponding hole/slit at the top of the tibial cutting guide to determine the rotational angle of the tibial implant [16].
After removing the templates, trial implants could be inserted for primary verification of the accuracy of the bone cuts and to adjust the soft tissue balance for the mediolateral and flexion–extension planes. Such trial implants are plastic (polyamide) and are fabricated in a manner similar to that used for producing the PST [17]. This technique could be used for simple, routine, straightforward TKAs, complex cases that are difficult to manage with the conventional technique, and those for which surgery is contraindicated because of an extra-articular deformity or retained hardware (e.g., nails, screws, plates) [6]. The PST is also useful for patients with bleeding tendencies (e.g., hemophiliacs), medically unfit patients at high risk of anesthesia, those with severe osteoarthritis combined with severe bone/cartilage loss, patients with complex articular anatomy, and those at increased risk of infection. The common feature of these patients is that conventional TKA would be difficult as it carries a risk of bleeding, infection, and prolonged operative time because of the use of intramedullary guides [18].
The first author’s experience with PST has focused on the CT-based technique, although the CT scans are performed using a tailored protocol. The drawback of this system is that it is limited to use in patients who previously underwent TKA on the contralateral knee that must now be bent during scanning to avoid interference. Nevertheless, CT-based software is easy to use because tissue segmentation is done automatically with the possibility for manual adjustment [3]. Also, it allows the surgeon to participate in the preoperative planning and to design the templates. (In contrast, MRI-based software requires an experienced technician to perform manual segmentation of the tissues.) The original clinical trials included patients with extra-articular deformities, a high bleeding tendency, bilateral deep vein thromboses, and/or a pulmonary embolism. They also included bilateral procedures for patients seeking speedy recovery of both limbs simultaneously and medically compromised individuals with cardiorespiratory problems. These patients had been denied conventional TKA by anesthetists or surgeons.
In each of these patients, PST was applied successfully according to the preoperative planning without resorting to conventional instrumentation, intramedullary guides, or alignment rods. Sizing was applied, as accurately predicted from preoperative planning, and a tourniquet (not drains) was used for all cases. Postoperatively, full extension and >100° flexion were achieved [13].
Preoperative anteroposterior and lateral view radiographs of the planned surgery were used for superimposing the implants. The surgeon was guided by screen shots or other visual aids while placing the templates and implants during surgery. The final plan was printed out and taken to the operating room for any verification required throughout the surgery. It gave full details about positioning and fixing the templates over bone for accurate implementation, double checking to avoid inaccurate positioning of the PST and subsequent inaccurate bone cutting, implant malpositioning, and limb malalignment. Placing each template over sound-receptive bone in a satisfactory matching manner took an average of 5 min [4]. It was longer for patients with a severe flexion deformity (>30°), where two femoral templates were designed to have a second option for excessive distal cutting and were used intraoperatively for optimal bone cutting [13].
Nearly all major implant companies currently use PSTs as standard technique. The PSTs were approved by the US Food and Drug Administration as Class 2 instruments. The major use of PSTs currently is as pin locators for conventional instruments. A set of conventional jigs is costly, however, as it contains more than 150–200 instruments, needs a special sterilization setup, and may cause staff confusion during surgery. This issue has long been a subject of debate as many authors believe that the main objective of PSTs is to replace conventional TKA instruments because the clinical applicability of PSTs has been confirmed [18]. Other studies criticized this technique and found that it provided suboptimal clinical results. For example, Klatt et al. used image-free navigation to evaluate PST bone cuts and postoperative alignment and found that there was >3° deviation from the mechanical axis [19]. The reason for this error was a faulty preoperative plan carried out on MRI software. Another report compared the results of custom-fit guides to traditional TKA and found a shorter operative time in 14% of the PST procedures and an average deviation from the mechanical axis by 1.2° varus [20]. Howell et al. documented 48 consecutive TKAs done with PSTs in which all subjects rapidly showed acceptable clinical outcomes (speedy recovery, well-restored motion, good stability, postoperative mechanical axis alignment, and high patient satisfaction). None of these patients required soft tissue release from the collateral or retinacular ligaments. Only three tibial and three femoral guides were improperly positioned owing to improper alignment on the preoperative planning using MRI software [15].
In the current setup, 3D printers had been modified to serve a variety of architects, surgeons, and dentists, among others, with specific features to produce complex structures. These modifications involved a variety of materials, each with differing physical and thermal properties and, consequently, different uses. It allowed the production of PSTs with a variety of materials (plastics), each with its specific biocompatibility, heat stability, sterilization method (autoclave or gamma rays), durability, and price. Other modifications in 3D printers included the addition of extruders or increasing the surface area of the printing bed to allow printing of larger or longer objects [8]. The manufacturers of these 3D printers were able to produce small, compact printers of the same size as paper printers. Called “desktop 3D printers,” they are easy to carry, can be placed in a room or office, and are less expensive than industrial 3D printers. These desktop 3D printers could be purchased by hospitals and stored in a laboratory room, imaging department, or even an outpatient clinic. Thus, imaging, preoperative planning, and PST production could be done in one workplace, thereby saving time and resources. In addition, 3D CT scanning tools with a cone beam have become a feasible tool to use in orthopedic surgery [21].
4.4.
New Features of PST
The development of PST for clinical use allowed new users to benefit from this approach. The learning curve starts with performing surgery on 3D-printed plastic knees according to a patient’s CT scan, thereby allowing the surgeon to predict the surgical results prior to operating on a living patient. Training involves positioning the PST over bony surfaces accurately and marking the level/inclination of bone cutting using conventional instruments or navigation guidance to compare and evaluate the cuts. The surgeon thus develops familiarity with the technique and gains confidence using it by seeing how simple instruments (e.g., angel wings) could be used to level bone cuts, visually inspect the surgical site, and confirm the appropriateness of the surgical procedure. After an average of five to ten cases, the surgeon should be ready to become involved in clinical, comparative trials [910].
PSTs are useful in active young patients who need to preserve good bony stock. Its advantage lies in the fact that the amount of bone to be cut is quantified before the actual surgery. Accurate preoperative planning for PSTs is most advantageous when applied to revision TKA, unicompartmental TKA, bicondylar replacement of only the medial and lateral compartments while preserving the anterior and posterior cruciate ligaments, patellofemoral arthroplasty, and high tibial osteotomy (Fig. 4.1) [22]. PSTs are also useful for hip resurfacing and total hip arthroplasty, comprising a powerful and inexpensive training tool.
Fig. 4.1
Patient-specific orthopedics
The currently used software provides a good opportunity for surgeons to participate and modify the preoperative planning of TKA and to learn how to interpret dynamic and kinematic data regarding the size, alignment, rotation, and virtual bone cutting. It also allows simulation of the surgery, which allows the surgeon to identify and analyze any error in the 3D planes in real time [9]. Moreover, this training improves cognitive and motor skills as it is based on repetitive practice while committing and correcting errors. Reusable 3D-printed plastic bones and PSTs are used in arthroplasty workshops. It has been noted that training for the PST technique for TKA is easier than that for the conventional technique as it involves fewer instruments and operative steps.
Medical and legal staff have referred to PST usage as a complex process, and surgeons are held responsible in case of failure as PST manufacturers cannot proceed with developing the procedure without obtaining acceptance from the surgeon [23]. For current company-based PST systems, planning is considered a serious limitation as it is done by technicians, not the surgeon, and the whole process is controlled by the implant company, with the surgeon able only to review the treatment plan. For example, improper fitting of a PST could cause overhang of a knee prosthesis, in turn causing irritation of the surrounding soft tissues. Alternatively, under-coverage could cause subsidence and instability [3].
PSTs offer the advantage of trial (simulated) surgery so the results of the surgical outcome could be foreseen and the surgeon can validate the accuracy, leveling, and inclination of the bone resection. PST technology is still in its early stages, and more work is needed to investigate the accurate placement of jigs over the bony surfaces [8]. This accurate placement demands proper soft tissue management. (For example, it is unprofitable to detach all soft tissues from bone to find an optimal fit for the PST.) When MRI is used for preoperative planning, cartilage thickness could cause inaccurate planning and subsequent placement of the PST. This is especially true for the tibial jig, which is usually weak and requires good assessment prior to the final placement and bone cutting. Inaccurate segmentation or planning would result in malpositioning of the PST and erroneous cuts in three dimensions [24].
The amount of intraoperative information on navigation and robotics is enormous. An average of 1° of deviation with navigation is considered accurate with regard to implant positioning and bony cuts. Verification is then done with much ease so changes could be made prior to the definitive cuts. The PST system, however, lacks verification tools, and minor malalignment with the risk of notching cannot be detected by the human eye. This disadvantage of PST has been well documented [25]. Thus, surgeons in training must verify their cuts by navigation or extramedullary rods in at least their first ten cases. This safeguard could significantly help junior surgeons understand the concept of PSTs and shorten the learning curve. The verification step could help these learners clarify their preferences regarding the usefulness of the various arthroplasty techniques, implant designs, and surgical approaches. The primary results of training usually show higher accuracy with femoral bone cutting, during which utmost attention must be focused on the tibial cutting position. Neophyte surgeons should not rely on medical representatives when choosing appropriate PST systems as the company representatives usually promise that their system is the most accurate [24].
The currently available PSTs are mostly produced by implant companies and are specific for their own knee prostheses. In contrast, the use of an open-platform technique allows coordination of the PST with any surgical instruments. This integration, or coupling, could be done by various methods based on connecting the custom-made guides to any instrument system from any implant manufacturer. The same concept can be applied for other joint arthroplasty procedures such as for the hip, shoulder, elbow, ankle, and others (Fig. 4.1) [13]. This technique is suitable for any commercially available instrument or prosthesis, and it gives the surgeons the option of using any implant and any instrumentation system. The technique involves different designs and shapes of PST to fit available instruments and implants. For example, using computer software, deep indents could be designed on the back of the cutting block to offer secure fitting [26]. Another design could be “housing,” created by interlocking the block from three sides and mounting it over a custom-made guide. To ensure accurate mounting, additional pins could be used to fix the cutting block over the custom-made guide. Several variations could be accomplished with the PST design using slots, grooves, or boxes with variable inclinations and angulations. These designs make PST more universal and applicable for patients with variable anatomy and deformities [4].
4.5.
Modern Applications of PST
Current training strategies to familiarize junior surgeons and nurses with PST systems vary among surgeons and hospitals. The first author introduced the self-contained, or hospital-based, PST system (Fig. 4.2). With this setup, imaging, planning, sizing, designing, and fabrication of the PST are completed by a technical team directly supervised by the operating surgeon [24]. Thus, the staff could communicate rapidly and easily, and the planned design could be instantly adjusted. This system is based on CT images because the waiting time is shorter and the cost of CT is less than that of MRI by an average of USD 40–300 in developing countries, Europe, and the United States. The overall cost of a PST in a hospital-based setup is nearly USD 500 and ranges from USD 800 to USD 2000 in company-based systems for cutting blocks or pin locators [15]. In addition to the cost element, the number of instruments used in a hospital-based system are 2 PSTs and 15 instruments, whereas for a company-based system, it is 2 PSTs and an average of 30–43 instruments, which is significantly more. It is worth mentioning that the fewer instruments needed, the less complicated is their sterilization and packing setup and the less disturbance there is in the operating room [16]. All of the steps for TKA in the hospital-based system can be carried out within the same workplace (the hospital), including the examination, imagining, planning, fabrication, packing, sterilization, surgery, and rehabilitation. There is no need for any company representative or special method for transferring data, radiographs, or documents.
Fig. 4.2
Hospital-based patient-specific template system for total knee arthroplasty. CT computed tomography, 3D three-dimensional, TKR total knee replacement
The one obstacle to applying hospital-based PST is the cost of the 3D printers (average USD 500,000) that are used to produce the templates. The currently available desktop 3D printers, however, have replaced industrial machines and are capable of producing the physical PST and trial implant models. These physical models are made of nylon, which is known to be heat stable, autoclavable, sufficiently durable to resist the force of saw blades, inexpensive, and easily manufactured within a short time and in easily available settings [24]. Another material used to produce PSTs is ABS plastics, which is also not expensive but requires sterilization with gamma rays, low-temperature steam, or other special chemicals. Another material that could be used is polycarbonate which could be sterilized by autoclave, gamma rays, or chemicals [89].
Another advantage of the PST is its applicability for bilateral one-stage TKA procedures. Simultaneous bilateral TKA is considered a good option for patients with bilateral knee osteoarthritis because it offers a single anesthesia exposure and simultaneous rehabilitation of both limbs with better functional outcome, although there remain some concerns about its safety. The Swedish Knee Arthroplasty Register and the Swedish Cause of Death Register showed a difference in the early mortality rate among patients treated with simultaneous bilateral TKA versus others who underwent staged procedures [27]. Previous reports showed that blood loss is one of the major complications associated with bilateral TKA. Because the risk of bleeding was 2.2 units per single TKA (caused by extensive osteotomy and soft tissue cuts), it would be logical to assume that treating two knees would double the risk of bleeding and the need for blood transfusion. During TKA, however, the need for blood transfusion has several predisposing factors, such as the surgical technique, operating time, and clotting factors. As computer-assisted orthopedic surgery techniques have the general advantages of avoiding intramedullary perforation and preserving soft tissues—which shorten the recovery time and reduce the rate of blood loss—PST is useful for simultaneous bilateral TKA while eliminating the need for intramedullary rods and subsequent bleeding, fat embolism, and infection. It is also useful for treating bilateral cases of severe articular deformities (valgus, varus, and fixed flexion deformity) during a single hospitalization, with a shorter accumulative operating time and, accordingly, less cost [28].
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13. The past, the present and the future. Chapter · January 2012. In book: Improving Accuracy in knee Arthroplasty, Chapter: 12, Publisher: Jaypee Brothers Medical Publishers (P) Ltd., Editors: Thienpont E., pp.149-168.

27. Anna Stefánsdóttir, Lars Lidgren, and Otto Robertsson. Higher Early Mortality with Simultaneous Rather than Staged Bilateral TKAs: Results From the Swedish Knee Arthroplasty Register. Clin Orthop Relat Res. 2008 Dec; 466(12): 3066–3070.

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