Patient Specific Instrumentation

Jessica Morton, MD

Ran Schwarzkopf, MD, MSc

Jonathan M. Vigdorchik, MD

BACKGROUND

Patient specific instrumentation (PSI) has undergone several iterations as technology has improved. Initially instruments were manufactured from three-dimensional imaging to accurately place pins for conventional cutting guides and preoperatively template the implant size. This has advanced to modern-day custom-engineered cutting guides through which bony resection can be performed as part of single-use disposable instrument trays. Unlike robotic or computer-navigated surgery, planning is performed preoperatively and no registration of bone position or computer system is necessary in the operating room.

The instruments are templated using three-dimensional imaging, computed tomography (CT) scan, or magnetic resonance imaging (MRI). Initially a three-dimensional model of the patient’s anatomy is created; subsequently, using templating software, bone resections are templated and components are sized. Surgeons review and edit the proposed resection and sizing of implants with the company technician. Finally, after surgeon approval using three-dimensional printing and layered resin the custom guides are created and sterilized for use.

The driving forces behind the creation of PSI is to improve accuracy of bone resections, increase reproducibility, reduce blood loss, improve clinical outcomes, decrease operative time, and lower costs by decreasing intraoperative decision making and reducing the number of instrument trays, and turnover time.¹ Studies have demonstrated the reliability, safety, and accuracy of PSI; however, they have failed to show improved patient outcomes and cost-efficiency over conventional TKA instrumentation, and controversy around this topic continues.

ALIGNMENT

Approximately 15% to 25% of patients who undergo primary TKA are not satisfied with their outcome.²,³,⁴ The source of that dissatisfaction remains debated, but deviations in mechanical alignment have been attributed as a potential source of this dissatisfaction.

While not in the scope of this chapter, there remains debate whether mechanical or kinematic alignment is the best method of component positioning.⁵ We will briefly discuss both types of alignment as it is important in understanding the radiographic outcomes and proposed uses of PSI.

Kinematic Alignment

Kinematic alignment strives to restore an individual’s alignment to its prearthritic state, using the kinematic axes of the knee. Kinematic alignment seeks to restore the native 3D alignment of the knee and three axes of normal knee motion. The specific goals of kinematic alignment are to restore native tibial-femoral articular surfaces, native knee and limb alignment, and native laxities.⁵,⁶,⁷ This native alignment can vary significantly between individuals and is proposed as a source of postoperative dissatisfaction after TKA. Using three-dimensional imaging to model conditions prior to onset of arthritis, restore native anatomy, and custom generate cutting guides allows physicians using PSI to recreate a patient’s kinematic alignment.

Mechanical Alignment

The mechanical axis of the lower extremity is the line drawn from the center of the femoral head to the center of the talus.⁸,⁹ The mechanical alignment of the femur is about 3° of valgus, and the tibia is generally aligned with the mechanical alignment of the limb.⁸ The postoperative restoration of limb mechanical axis within 3° of neutral has been an established predictor of success in TKA.⁸,¹⁰,¹¹,¹²,¹³ Implant malalignment has been reported as high as 20% to 40%,¹⁴ and outliers, those greater than 3° from neutral, are at a higher risk of failure. Malalignment exceeding 3° of varus or valgus in the coronal plane increases aseptic loosening of implants, at a rate of 24% compared with a rate of 3% in patients within 3° of neutral mechanical axis. Malalignment can also lead to accelerated polyethylene wear and medial bone collapse.⁹

Establishing a patient’s mechanical axis requires long-leg radiographs to accurately measure the hip-knee-ankle (HKA) angle, tibia mechanical axis (TMA), femoral mechanical axis (FMA). Although there are studies that demonstrate correlation between mechanical and anatomic axis, measured on short-leg radiographs, long-leg radiographs are the gold standard¹⁵,¹⁶,¹⁷ (Fig. 33-1).

Hip-Knee-Ankle Angle

Despite extensive research and several meta-analyses on the topic there is no significant difference in outliers of greater than 3° from neutral mechanical axis in the hip-knee-ankle angle when comparing conventional instrumentation and PSI.¹⁸,¹⁹,²⁰,²¹,²²

FIGURE 33-1 The mechanical axis (A), femoral mechanical axis (FMA) (B), tibial mechanical axis (TMA) (C), and hip-knee-ankle angle (HKA) (D) in a patient undergoing preoperative evaluation. Mechanical axis of the knee is measured from the center of the femoral head to the center of the talus. In this patient the line passes medial to the center of the knee demonstrating a varus deformity (A). FMA is defined as a line from the center of the femoral head to the center of the intercondylar notch (B). TMA is defined as a line from the center of the tibia to the talus (C). The HKA is defined as the angle between the TMA and the FMA (D).

Coronal Alignment

Coronal alignment of the tibial and femoral components is key to component positioning and successful TKA. Multiple studies have reported on tibial component coronal alignment with mixed results. In metanalysis both Thienpont et al and Zhang et al found significantly increased risk of malalignment,²⁰,²² while Cavaignac et al. fond no significant difference.¹⁸ Femoral component coronal alignment was slightly favored in metanalysis of PSI but infrequently reached statistical significance.¹⁸,¹⁹,²⁰,²¹,²²

Sagittal Alignment

Sagittal alignment remains more difficult to judge and is infrequently evaluated or reported. Flexion of the femoral component greater than 3° or sagittal alignment of the tibial component less than neutral, or tibial slope greater than 7° has been identified as a risk factor for failure.²³ Sagittal instability can occur secondary to this malalignment. While infrequently evaluated in the literature, a meta-analysis by Zhang et al found more tibial slope outliers when using PSI rather than conventional instrumentation²² and Thienpont et al found a significantly higher probability of malalignment of the tibial component in the sagittal plane with use of PSI, although neither of these studies reached statistical significance.²⁰,²² In a multicenter, randomized, clinical trial, Boonen et al reported a significant increase in outliers in the sagittal plane of the femoral component when using custom cutting guides.²⁴

TEMPLATING AND PREOPERATIVE PLANNING

Preoperative planning for PSI is system dependent, but generally starts with three-dimensional imaging with or without long-leg radiographs to generate a working model of the knee. A proposed surgical plan with bone resection and implant sizing is provided to the surgeon who makes adjustments and returns the plan to the manufacturer. The cutting guides are then created offsite and sent to the surgeon for intraoperative use. Figs. 33-2 and 33-3 demonstrate three-dimensional models of the knee with custom printed resection guides.

MANUFACTURER

There are several PSI total knee systems commercially available at this time, each with its accompanying template generation software and prosthetic options. Average lead time needed to generate cutting guides varies from
18 to 30 days.²⁵ Table 33-1 lists some of the currently available PSI systems, preferred imaging modality, and average lead time in working days.

FIGURE 33-2 Coronal (A) and sagittal (B) views of the three-dimensional model of the tibia with custom cutting guide.

CT VERSUS MRI

The superior imaging modality for the creation of modeling and cutting guides remains debated. CT is less expensive with shorter imaging time, however does include the risk of increased radiation despite low-dose protocols. Radiation remains equivalent to a standard yearly background radiation dose or approximately 70 chest X-rays.²⁶ MRI does not utilize ionizing radiation; however, protocols often require the concurrent use of full-leg standing radiographs. MRI is a longer study and in most health systems more expensive. MRI has the advantage of providing additional information regarding soft tissue structures including ligaments and residual cartilage thickness which can influence guide fit and cut thickness.²⁰

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