Patient-Specific Instrumentation in Arthroplasty

Patient-specific instrumentation (PSI) — also termed custom cutting blocks, patient-matched instrumentation, or patient-specific guides — uses pre-operative CT or MRI imaging of the patient`s own anatomy to manufacture single-use custom cutting jigs that are designed to fit precisely onto the patient`s bone surface. The guides eliminate the need for standard intramedullary or extramedullary jig systems, theoretically improving the accuracy of bone cuts, component alignment, and surgical efficiency. PSI was introduced for TKA around 2007–2010 and subsequently for THA and UKA. Unlike robotic and navigation systems, PSI does not provide intraoperative feedback — the accuracy depends entirely on the pre-operative plan and the quality of fit of the guides to the bone surface on the day of surgery.

• Manufacturers and systems: Visionaire (Smith & Nephew) — MRI-based; OtisKnee (Exactech) — CT-based; Signature (Biomet) — MRI-based; TruMatch (DePuy) — CT-based; Prophecy (Wright Medical) — CT-based, used primarily for ankle arthroplasty; each system has its own imaging protocol, planning software, and guide design
• How PSI works: (1) Pre-operative imaging (CT or MRI of the full limb or the affected joint); (2) Virtual planning — the imaging data are processed by the manufacturer`s planning software to create a 3D bone model; the surgeon reviews and approves the implant size, position, and alignment on the virtual model; (3) Guide manufacture — custom cutting jigs are manufactured (typically from nylon or similar medical-grade polymer) to precisely match the contour of the patient`s bone surface; (4) Intraoperative use — the guides are applied to the exposed bone surface; correct seating of the guide on the bone surface is confirmed before cutting; the guide slots indicate the direction and level of bone cuts
• Claimed advantages: elimination of intramedullary canal violation (no IM rod in the femur or tibia — reduces blood loss, fat embolism risk, and infection risk from canal violation); reduced instrument inventory (fewer standard trays required in theatre — potential efficiency and cost saving in instrument sterilisation); potential for improved alignment accuracy; operative time reduction (once the guide seating learning curve is overcome)

• The evidence for PSI in TKA is mixed and largely disappointing compared to the initial promise: multiple RCTs and meta-analyses have found that PSI does NOT consistently improve coronal alignment accuracy compared to conventional TKA; a 2015 Cochrane systematic review (Thienpont et al.) and multiple meta-analyses (Voleti et al.; Fu et al.) found no statistically significant difference in the proportion of outliers in coronal alignment (HKA angle outside 180° ± 3°) between PSI and conventional TKA; some studies have shown marginal improvements in sagittal alignment and tibial component rotation with PSI, but these are inconsistent
• Why does PSI not improve alignment reliably: the accuracy of PSI is critically dependent on the quality of guide seating on the bone surface; articular cartilage wear, osteophytes, and soft tissue impingement can prevent the guide from seating correctly on its planned surface — the guide may appear to seat adequately but be malpositioned by several millimetres or degrees; this `seating error` is the primary cause of PSI inaccuracy; the planned position is only achieved if the guide sits exactly where it was designed to sit, which cannot be verified intraoperatively without navigation or imaging feedback
• Operative time: the expected reduction in operative time with PSI has not been consistently realised in RCTs; several studies found no significant difference in operative time between PSI and conventional TKA; some studies found PSI was actually slower (due to the learning curve for correct guide seating verification); the promised theatre efficiency gains have not been clearly demonstrated
• Blood loss: the elimination of IM canal violation was expected to reduce blood loss; some studies have shown modest reductions in blood loss with PSI (no IM rod violation); however, the clinical significance is questionable given that tranexamic acid has largely eliminated clinically significant blood loss in modern TKA regardless of technique

• Where PSI has the most potential: PSI offers the greatest potential benefit in cases where standard jig systems are unreliable — prior femoral or tibial osteotomy (distorted medullary canal preventing IM rod insertion); post-trauma deformity with distorted anatomy; revision TKA (distorted or absent normal landmarks); severe extra-articular deformity; in these complex cases, a custom guide designed around the actual bone anatomy provides a more reliable cutting reference than a standard jig applied to an abnormal bone
• THA PSI: custom acetabular positioning guides have been developed for THA; the guide matches the acetabular rim shape and provides a specific reaming direction and cup orientation; evidence is limited but early data suggest improved cup positioning consistency; particularly useful for complex primary THA (developmental dysplasia of the hip, post-fracture deformity) and revision THA
• PSI for ankle arthroplasty: the Prophecy PSI system for total ankle replacement (STAR implant, INFINITY implant) is one of the most validated PSI applications; the ankle joint`s small size, complex geometry, and high sensitivity to component malalignment make PSI potentially more impactful here than in TKA; CT-based Prophecy guides have demonstrated improved coronal and sagittal tibial component alignment compared to conventional ankle arthroplasty guides in multiple studies

• PSI vs navigation vs robotics — choosing the technology: PSI is the simplest and most affordable of the three technologies; it requires no intraoperative hardware or capital equipment (just the pre-manufactured guide); however, it provides no intraoperative feedback — if the guide seats incorrectly, the error is not detected; navigation provides intraoperative feedback and can detect and correct errors in real time but requires capital investment and adds operative time; robotic systems provide the highest level of intraoperative precision and soft tissue balance assessment but have the highest cost and operative time addition; the appropriate technology depends on the case complexity, available resources, and surgeon preference
• Lead time and logistical considerations: PSI requires a minimum lead time of 3–6 weeks between imaging and surgery (for imaging processing and guide manufacture); surgery cannot be brought forward without new guides; if the implant sizing changes intraoperatively (unexpected finding), the PSI guide may not be appropriate for the revised implant size; a standard instrument tray must always be available as a backup; delays in guide delivery or guide manufacturing errors require postponement of surgery — logistical robustness is essential for a PSI programme
• The future of PSI: the most likely evolution of PSI is as a planning tool combined with robotic execution — the pre-operative CT planning and virtual templating inherent in PSI workflows feeds naturally into robotic surgical systems; the standalone PSI cutting guide may be superseded by robotic systems that use the same pre-operative planning data to guide the robotic arm intraoperatively, combining the planning benefits of PSI with the intraoperative feedback of robotics

• PSI: custom cutting guides manufactured from patient`s CT or MRI; fits precisely to bone surface; eliminates standard IM jig systems; NO intraoperative feedback — accuracy depends entirely on correct guide seating
• Workflow: pre-op CT/MRI → 3D virtual planning (surgeon approves) → guide manufacture (3–6 weeks lead time) → intraoperative guide application → bone cuts through guide slots
• Evidence summary: PSI does NOT consistently improve coronal alignment in TKA vs conventional (Cochrane 2015; multiple meta-analyses); seating error is the primary failure mode; operative time not reliably reduced; blood loss advantage marginal in era of TXA
• Seating error: articular cartilage wear, osteophytes, soft tissue prevent accurate guide seating; guide appears seated but is malpositioned; no intraoperative verification possible without navigation
• Best applications for PSI: complex cases — prior osteotomy, post-trauma deformity, severe extra-articular deformity, revision TKA; standard jigs unreliable; PSI designed to actual (abnormal) anatomy; also ankle arthroplasty (Prophecy system)
• PSI vs navigation vs robotics: PSI — cheapest, no intraoperative feedback; navigation — intraoperative feedback, capital cost, adds time; robotics — highest precision + soft tissue balance, highest cost; choose based on case complexity and resources
• Lead time: 3–6 weeks minimum; cannot bring surgery forward; standard instrument tray always required as backup; guide manufacturing errors cause postponement
• Ankle PSI (Prophecy): best validated PSI application; CT-based; improved tibial component alignment in total ankle replacement; ankle`s small size and high alignment sensitivity makes PSI more impactful than in TKA
• Future: PSI as planning tool feeding into robotic execution; standalone PSI guide likely superseded by integrated robotic planning + execution systems