ORTHOPAEDICS ANZJSurg.com
Operative efficiency and accuracy of patient-specific cutting guides in total knee replacement Mark Nankivell, Graeme West and Nicholas Pourgiezis Orthopaedics and Trauma, The Queen Elizabeth Hospital, Adelaide, South Australia, Australia
Key words arthroplasty, knee, outcome, patient-specific, replacement. Correspondence Dr Mark Nankivell, Orthopaedics and Trauma, The Queen Elizabeth Hospital, 28 Woodville Road, Woodville, SA 5011, Australia. Email: [email protected] M. Nankivell BMBS; G. West BPhysio; N. Pourgiezis BMedSci, MBBS, FRACS, FAOrthA. Accepted for publication 27 September 2014. doi: 10.1111/ans.12906
Abstract Background: Total knee replacement (TKR) outcomes depend on accurate positioning of implants and restoration of the mechanical axis of the knee. Compared with standard techniques, patient-specific cutting guides are postulated to improve accuracy of bone resections, and therefore implant placement. Furthermore, patient-specific cutting guides are postulated to reduce operative time and increase efficiency by reducing the number of trays used. Methods: This study evaluates these claims using the Visionaire (Smith & Nephew, Inc., Memphis, TN, USA) patient-specific system. The thickness of actual bone resections was compared with the predicted thickness (giving a resection ‘error’). Data were also obtained on the number of trays used, skin-to-skin operating time and tourniquet time. Results: Forty-one TKRs were performed on 33 females (one bilateral) and seven males. Average resection errors were 0.22 mm medially and 0.05 mm laterally for the distal femur, 0.99 mm medially and 0.74 mm laterally for posterior femoral condyles, and 0.55 mm medially and 0.71 mm laterally for the proximal tibia. There were no significant differences in tourniquet time, skin-to-skin time or the number of trays used between the patient-specific and historical comparison groups. Conclusion: Patient-specific cutting guides make accurate resections. Operative and tourniquet times and the number of trays used were no different to standard TKRs. Further investigation is needed to determine whether patient-specific cutting guides improve post-operative alignment and patient satisfaction.
Introduction Total knee replacement (TKR) is the gold standard treatment for knee arthritis, affording patients pain relief and restoration of function.1 Axial alignment within ±3° varus/valgus is associated with more optimal outcome than implant alignment outside of this range.2 However, studies have shown up to 25%2 or even 30%3 of TKRs have alignment outside of this preferred range. It is believed that implant malalignment leads to altered biomechanical forces and increased implant wear and failure.2 Therefore, accurate bone resection is paramount in optimizing implant positioning and ultimately clinical outcome. Alignment aids have been developed to increase reliability and accuracy of implant positioning. Such guides have limitations however, and errors can be made because of variations in bony anatomy, limitations in technique or misjudgement of visual landmarks by surgeons.3 Standard instrumentation has built-in assumptions of bone geometry, which may lead to errors of implant positioning.4 ANZ J Surg 85 (2015) 452–455
Computer navigation has been developed in an attempt to overcome some of these issues. However, some authors have found numerous shortcomings of computer navigation, including longer operative times and a long learning curve; difficulties and mistakes with landmark registration; pin site issues including loosening, fracture and infection; and the cost and bulk of the equipment.1 Patient-specific cutting guides (PSCGs) have been developed to aid optimal implant positioning.5 PSCGs use three-dimensional imaging to customize the bone resections and to size and position the implants. In their prospective randomized study of 29 patients undergoing primary TKR using either magnetic resonance imaging (MRI)-based PSCGs (Visionaire, Smith & Nephew, Inc., Memphis, TN, USA) or standard instrumentation, Noble et al. found significantly improved alignment in the PSCG group, along with reductions in operative time, length of hospital stay, incision length and number of trays used.5 Other authors have suggested that these efficiencies are cost beneficial because of decreased costs of sterilization and handling, © 2014 Royal Australasian College of Surgeons
Accuracy of patient-specific cutting guides in TKR
and reduced set-up times, and that these gains in efficiency offset the increased cost of acquiring three-dimensional imaging and PSCGs.6 Biant et al. have assessed the accuracy of computer navigated bone resection. They found that the physical space left by the bone resection was, on average, within 0.4 mm of that calculated. They concluded that this was accurate and leads to reliable resections.7 To date, no authors have examined the accuracy of MRI-based PSCG bone resections. This study aims to assess the accuracy of these bone resections, and to evaluate if these guides lead to increased operative efficiency as suggested by Noble.5
Methods Patients were recruited through our hospital orthopaedic outpatient clinics. Patients were included if they were assessed as candidates for TKR with knee pain as a result of osteoarthritis, rheumatoid arthritis or post-traumatic arthritis; who need correction of varus, valgus or post-traumatic deformity; were of legal age, able to consent and had reached skeletal maturity; were of good nutritional state; and who were able and willing to undergo an MRI scan. Patients were excluded if they had signs of infection, osteomyelitis or sepsis; had previous joint replacement of that knee; were skeletally immature, pregnant, unable to undergo MRI or severely overweight (body mass index (BMI) >40); or if they had physical, emotional or neurological conditions that could compromise their ability to comply with post-operative rehabilitation and follow-up. Approval was granted by our hospital’s ethics committee. The patients were consented to undergo TKR using the Visionaire PSCGs and Genesis II TKR implants (Smith & Nephew, Inc.). The Visionaire system uses MRI and long-leg XR data to design each cutting guide according to proprietary algorithms. Patient demographics (age, gender, side of operation and BMI) were collected at the time of recruitment. Post-operative alignment data was also collected and forms the basis of another study. All surgeries were done by consultant orthopaedic surgeons using the surgeon’s standard medial parapatellar approach. The Visionaire cutting blocks were attached to the distal femur and proximal tibia without removing osteophytes as per the manufacturer’s standard procedure. Cuts were then made through the guides. The Visionaire femur block is designed to make the initial distal femur cut, with the remaining femoral cuts utilizing the standard cutting block. Resection of the proximal tibia is made through the tibial PSCG. The resected bone was then measured with a micrometer at the thickest points. The distal femur resections were measured medially and laterally, thus obtaining distal femur medial resection and distal femur lateral resection measurements. The posterior femoral condyle cuts were made with the standard cutting block; the resultant medial posterior femoral condyle (posteromedial resection) and lateral posterior femoral condyle (posterolateral resection) resections were also measured. The proximal tibia resection was made through the Visionaire tibial block, and the resultant resection was measured medially (medial tibial resection) and laterally (lateral tibial resection). Each of these measurements was made three times and averaged. The thickness of the saw blade was added to the resection thickness to calculate total resection for each cut. The micrometer used was accurate to increments of 0.5 mm. © 2014 Royal Australasian College of Surgeons
453
Table 1 Summary of demographics, skin incision to skin closing duration (skin time), duration of tourniquet inflation, and the number of trays used of the PSCG and comparison groups Demographic No of TKRs Male : female Average age years (±SD) BMI average Side L : R Median ASA score Average skin time (min) ± SD Average tourniquet time (min) ± SD Average no. of trays used
PSCG
Comparison
41 7:33 70.8 (±9.6) 33.7 17:24 2 84 ± 15 82 ± 18 3.5
45 18:27 71.4 (±8.7) 32.8 26:19 2 88 ± 18 85 ± 17 4.1
ASA, American Society of Anesthesiologists; BMI, body mass index; PSCG, patient-specific cutting guide; SD, standard deviation; TKR, total knee replacement.
Table 2 Mean ‘resection error’ (predicted minus actual resection thicknesses; in millimetres) Resection
Femur
Tibia
Mean resection error in mm (±SD) Distal medial Distal lateral Posteromedial Posterolateral Medial Lateral
0.22 mm 0.05 mm 0.99 mm 0.74 mm 0.71 mm 0.55 mm
(±0.8) (±0.8) (±1.5)* (±0.82)** (±0.99)** (±1.18)***
*P = 0.001; **P = 0.0001; ***P = 0.005. SD, standard deviation.
The time from skin incision to skin closure (skin-to-skin) time, and from inflation to deflation of the tourniquet, was collected. The number of trays used was also counted. Results were compared with historical local data. The results were analysed by a third-party statistician using independent samples t-test for descriptive statistics. A Bland–Altman analysis was used to assess the level of agreement between the expected resection thickness on the basis of the MRI and the actual resection thickness obtained. A range of agreement was defined as mean bias ±2 standard deviation, producing a lower level of agreement and an upper level of agreement, meaning that 95% of cases fell within this band. A paired t-test was performed to test whether the bias was significant. The results were further interrogated via regression analysis to determine whether patient demographic (age, gender, side, BMI) had significant influence on the accuracy of the Visionaire jigs. Comparison data was obtained from the most recent 45 TKRs done in our hospital by the same surgeons using the same implants with conventional techniques. Data was obtained on patient demographics, surgical times and number of trays used.
Results Forty-five consecutive patients were recruited into the study; however, resection data was not available for the first four. Thus, 41 knees belonging to 40 patients were included in this study; 33 were females (one having both knees done during this study) and seven
454
Nankivell et al.
Fig. 1. Histograms of predicted minus actual resection thickness (difference in mm on x axis) for the 41 patient-specific cutting guide (PSCG) total knee replacement (TKRs): (a) distal femur medial resection, (b) distal femur lateral resection, (c) posterior femoral condyle medial resection, (d) posterior femoral condyle lateral resection, (e) proximal tibia medial resection, (f) proximal tibia lateral resection.
males. Their average age was 70.8 (±9.6) years, with an average BMI of 33.7 and a median American Society of Anesthesiologists (ASA) Score of 2. Seventeen operations were left sided and 24 were right. There were no significant differences between the PSCG and comparison groups with respect to patient demographics, skin-toskin time, tourniquet time and the number of trays used (Table 1). On further analysis, patient demographics (gender, age, side of operation and BMI) had no significant influence on the accuracy of the resections. The mean (± standard deviation) differences between predicted and actual resection thicknesses (‘resection error’) are shown in Table 2. The resection error of the distal femur (0.22 mm medial and 0.05 mm lateral) did not reach statistical significance. The resection error of the posterior femoral condyles (0.99 mm medial and
0.74 mm lateral) and the proximal tibia (0.71 mm medial and 0.55 mm lateral) did reach statistical significance. Histograms of these results have been presented in Figure 1.
Discussion This study shows that MRI-based PSCGs are accurate to within 1 mm on average. The other postulated benefits of PSCGs (reduced operating time, reduced tourniquet time and reduced number of trays) have however not been supported in this present study.
Accuracy Resections of the distal femur were within ±0.22 mm of predicted value. Thus, we are satisfied that the PSCGs cut where they are © 2014 Royal Australasian College of Surgeons
Accuracy of patient-specific cutting guides in TKR
Fig. 2. Line chart of number of trays used. The trend line shows that as the study progressed, fewer trays tended to be used. Note that no data are available for two cases (cases 11 and 38) as no record was made of the number of trays used.
planned. However, it is worth noting that greater variation was noted in the posterior femoral condyle resections, which are made through the standard cutting guide not the PSCG (however, the positioning of the standard cutting block is determined by PSCG). This implies the potential for loss of accuracy of the implanted femoral component, particularly with respect to rotation. This current study did not assess final implant positioning, which is currently being assessed by our team and forms the basis of a further study currently in progress.8 Furthermore, data was not collected on the number of knees requiring recuts for ‘soft tissue balancing’, and TKR stability, particularly flexion and extension stability, was not formally assessed in this paper.
Skin time and tourniquet time The consultant surgeons involved in this study performed the surgery using their own standard approach. Accordingly, some deflate the tourniquet before closing in order to achieve haemostasis, some leave the tourniquet inflated for the duration of the procedure. Furthermore, a tourniquet was not used in one case because of the presence of severe peripheral vascular disease. In another case, the tourniquet was deflated after 24 min (after soft tissue approach and bony resection, but before positioning of implants). There being no standardized approach to using a tourniquet in this study no firm conclusions about tourniquet time can be made. An average tourniquet time of 82 and skin-to-skin time of 84 min is comparable with our experience with conventional TKRs (Table 1).
Trays and cost-effectiveness Trays of instruments and implant trials all take time to handle and bear a cost associated with sterilization and packaging. Therefore, it has been proposed by others5 that if PSCGs can reduce the number of trays used, they will increase theatre (and sterilization department) efficiency and reduce costs to the health system. We did not collate patient theatre time or set-up time. In their study, Nunley et al. found no significant reduction in operating time but did find
© 2014 Royal Australasian College of Surgeons
455
theatre time reduced by 12 min per case. They suggested this reflected reduced set-up time (and therefore some reduced cost).6 In this study, we did not find any significant difference in skin time, tourniquet time or the number of trays used compared with conventional TKRs (Table 1). Therefore, claims of increased theatre (and health system) efficiency are not supported by our results. However, this study was undertaken early in the process of our surgeons’ use of PSCGs, and therefore, our theatre nursing staff had relative inexperience with the trays and set ups. The range in number of trays used (from two to seven) shows this variability. One case involved the opening of three wrong-sized trays necessitating the opening of three more and a further tray for patella resurfacing (thus a total of seven trays). Over the course of this study, less of trays were required (see the trend line in Figure 2), suggestive of a learning curve effect.
Conclusion It is important that our tools do what we think they do. This study measures the accuracy of PSCGs; however, this does not show evidence of the other purported benefits of PSCGs (e.g. increased efficiency). Furthermore, this paper has not assessed the accuracy of final prosthesis positioning as a result of the use of PSCGs. This is an area requiring further investigation. Finally, perhaps of more significance to patients is their satisfaction in terms of pain and function. More work needs to be done to determine if PSCGs lead to beneficial clinical outcomes (improved function and greater patient satisfaction) compared with conventional TKRs.
References 1. Lombardi AV Jr, Berend KR, Adams JB. Patient-specific approach in total knee arthroplasty. Orthopedics 2008; 31: 927–30. 2. Bali K, Walker P, Bruce W. Custom-fit total knee arthroplasty: our initial experience in 32 knees. J. Arthroplasty 2012; 27: 1149–54. 3. Bathis H, Perlick L, Tingart M, Luring C, Zurakowski D, Grifka J. Alignment in total knee arthroplasty. A comparison of computer-assisted surgery with the conventional technique. J. Bone Joint Surg. Br. 2004; 86: 682–7. 4. Howell SM, Kuznik K, Hull ML, Siston RA. Results of an initial experience with custom-fit positioning total knee arthroplasty in a series of 48 patients. Orthopedics 2008; 31: 857–63. 5. Noble JW Jr, Moore CA, Liu N. The value of patient-matched instrumentation in total knee arthroplasty. J. Arthroplasty 2012; 27: 153–5. 6. Nunley RM, Ellison BS, Ruh EL et al. Are patient-specific cutting blocks cost-effective for total knee arthroplasty? Clin. Orthop. Relat. Res. 2012; 470: 889–94. 7. Biant LC, Yeoh K, Walker PM, Bruce WJM, Walsh WR. The accuracy of bone resections made during computer navigated total knee replacement. Do we resect what the computer plans we resect? Knee 2008; 15: 238–41. 8. Pourgiezis N, Putalupattu S. Alignment and component position in patient-specific total knee arthroplasty: a prospective comparative study using Perth CT protocol (pending publication). 2014.
Operative efficiency and accuracy of patient-specific cutting guides in total knee replacement Mark Nankivell, Graeme West and Nicholas Pourgiezis Orthopaedics and Trauma, The Queen Elizabeth Hospital, Adelaide, South Australia, Australia
Key words arthroplasty, knee, outcome, patient-specific, replacement. Correspondence Dr Mark Nankivell, Orthopaedics and Trauma, The Queen Elizabeth Hospital, 28 Woodville Road, Woodville, SA 5011, Australia. Email: [email protected] M. Nankivell BMBS; G. West BPhysio; N. Pourgiezis BMedSci, MBBS, FRACS, FAOrthA. Accepted for publication 27 September 2014. doi: 10.1111/ans.12906
Abstract Background: Total knee replacement (TKR) outcomes depend on accurate positioning of implants and restoration of the mechanical axis of the knee. Compared with standard techniques, patient-specific cutting guides are postulated to improve accuracy of bone resections, and therefore implant placement. Furthermore, patient-specific cutting guides are postulated to reduce operative time and increase efficiency by reducing the number of trays used. Methods: This study evaluates these claims using the Visionaire (Smith & Nephew, Inc., Memphis, TN, USA) patient-specific system. The thickness of actual bone resections was compared with the predicted thickness (giving a resection ‘error’). Data were also obtained on the number of trays used, skin-to-skin operating time and tourniquet time. Results: Forty-one TKRs were performed on 33 females (one bilateral) and seven males. Average resection errors were 0.22 mm medially and 0.05 mm laterally for the distal femur, 0.99 mm medially and 0.74 mm laterally for posterior femoral condyles, and 0.55 mm medially and 0.71 mm laterally for the proximal tibia. There were no significant differences in tourniquet time, skin-to-skin time or the number of trays used between the patient-specific and historical comparison groups. Conclusion: Patient-specific cutting guides make accurate resections. Operative and tourniquet times and the number of trays used were no different to standard TKRs. Further investigation is needed to determine whether patient-specific cutting guides improve post-operative alignment and patient satisfaction.
Introduction Total knee replacement (TKR) is the gold standard treatment for knee arthritis, affording patients pain relief and restoration of function.1 Axial alignment within ±3° varus/valgus is associated with more optimal outcome than implant alignment outside of this range.2 However, studies have shown up to 25%2 or even 30%3 of TKRs have alignment outside of this preferred range. It is believed that implant malalignment leads to altered biomechanical forces and increased implant wear and failure.2 Therefore, accurate bone resection is paramount in optimizing implant positioning and ultimately clinical outcome. Alignment aids have been developed to increase reliability and accuracy of implant positioning. Such guides have limitations however, and errors can be made because of variations in bony anatomy, limitations in technique or misjudgement of visual landmarks by surgeons.3 Standard instrumentation has built-in assumptions of bone geometry, which may lead to errors of implant positioning.4 ANZ J Surg 85 (2015) 452–455
Computer navigation has been developed in an attempt to overcome some of these issues. However, some authors have found numerous shortcomings of computer navigation, including longer operative times and a long learning curve; difficulties and mistakes with landmark registration; pin site issues including loosening, fracture and infection; and the cost and bulk of the equipment.1 Patient-specific cutting guides (PSCGs) have been developed to aid optimal implant positioning.5 PSCGs use three-dimensional imaging to customize the bone resections and to size and position the implants. In their prospective randomized study of 29 patients undergoing primary TKR using either magnetic resonance imaging (MRI)-based PSCGs (Visionaire, Smith & Nephew, Inc., Memphis, TN, USA) or standard instrumentation, Noble et al. found significantly improved alignment in the PSCG group, along with reductions in operative time, length of hospital stay, incision length and number of trays used.5 Other authors have suggested that these efficiencies are cost beneficial because of decreased costs of sterilization and handling, © 2014 Royal Australasian College of Surgeons
Accuracy of patient-specific cutting guides in TKR
and reduced set-up times, and that these gains in efficiency offset the increased cost of acquiring three-dimensional imaging and PSCGs.6 Biant et al. have assessed the accuracy of computer navigated bone resection. They found that the physical space left by the bone resection was, on average, within 0.4 mm of that calculated. They concluded that this was accurate and leads to reliable resections.7 To date, no authors have examined the accuracy of MRI-based PSCG bone resections. This study aims to assess the accuracy of these bone resections, and to evaluate if these guides lead to increased operative efficiency as suggested by Noble.5
Methods Patients were recruited through our hospital orthopaedic outpatient clinics. Patients were included if they were assessed as candidates for TKR with knee pain as a result of osteoarthritis, rheumatoid arthritis or post-traumatic arthritis; who need correction of varus, valgus or post-traumatic deformity; were of legal age, able to consent and had reached skeletal maturity; were of good nutritional state; and who were able and willing to undergo an MRI scan. Patients were excluded if they had signs of infection, osteomyelitis or sepsis; had previous joint replacement of that knee; were skeletally immature, pregnant, unable to undergo MRI or severely overweight (body mass index (BMI) >40); or if they had physical, emotional or neurological conditions that could compromise their ability to comply with post-operative rehabilitation and follow-up. Approval was granted by our hospital’s ethics committee. The patients were consented to undergo TKR using the Visionaire PSCGs and Genesis II TKR implants (Smith & Nephew, Inc.). The Visionaire system uses MRI and long-leg XR data to design each cutting guide according to proprietary algorithms. Patient demographics (age, gender, side of operation and BMI) were collected at the time of recruitment. Post-operative alignment data was also collected and forms the basis of another study. All surgeries were done by consultant orthopaedic surgeons using the surgeon’s standard medial parapatellar approach. The Visionaire cutting blocks were attached to the distal femur and proximal tibia without removing osteophytes as per the manufacturer’s standard procedure. Cuts were then made through the guides. The Visionaire femur block is designed to make the initial distal femur cut, with the remaining femoral cuts utilizing the standard cutting block. Resection of the proximal tibia is made through the tibial PSCG. The resected bone was then measured with a micrometer at the thickest points. The distal femur resections were measured medially and laterally, thus obtaining distal femur medial resection and distal femur lateral resection measurements. The posterior femoral condyle cuts were made with the standard cutting block; the resultant medial posterior femoral condyle (posteromedial resection) and lateral posterior femoral condyle (posterolateral resection) resections were also measured. The proximal tibia resection was made through the Visionaire tibial block, and the resultant resection was measured medially (medial tibial resection) and laterally (lateral tibial resection). Each of these measurements was made three times and averaged. The thickness of the saw blade was added to the resection thickness to calculate total resection for each cut. The micrometer used was accurate to increments of 0.5 mm. © 2014 Royal Australasian College of Surgeons
453
Table 1 Summary of demographics, skin incision to skin closing duration (skin time), duration of tourniquet inflation, and the number of trays used of the PSCG and comparison groups Demographic No of TKRs Male : female Average age years (±SD) BMI average Side L : R Median ASA score Average skin time (min) ± SD Average tourniquet time (min) ± SD Average no. of trays used
PSCG
Comparison
41 7:33 70.8 (±9.6) 33.7 17:24 2 84 ± 15 82 ± 18 3.5
45 18:27 71.4 (±8.7) 32.8 26:19 2 88 ± 18 85 ± 17 4.1
ASA, American Society of Anesthesiologists; BMI, body mass index; PSCG, patient-specific cutting guide; SD, standard deviation; TKR, total knee replacement.
Table 2 Mean ‘resection error’ (predicted minus actual resection thicknesses; in millimetres) Resection
Femur
Tibia
Mean resection error in mm (±SD) Distal medial Distal lateral Posteromedial Posterolateral Medial Lateral
0.22 mm 0.05 mm 0.99 mm 0.74 mm 0.71 mm 0.55 mm
(±0.8) (±0.8) (±1.5)* (±0.82)** (±0.99)** (±1.18)***
*P = 0.001; **P = 0.0001; ***P = 0.005. SD, standard deviation.
The time from skin incision to skin closure (skin-to-skin) time, and from inflation to deflation of the tourniquet, was collected. The number of trays used was also counted. Results were compared with historical local data. The results were analysed by a third-party statistician using independent samples t-test for descriptive statistics. A Bland–Altman analysis was used to assess the level of agreement between the expected resection thickness on the basis of the MRI and the actual resection thickness obtained. A range of agreement was defined as mean bias ±2 standard deviation, producing a lower level of agreement and an upper level of agreement, meaning that 95% of cases fell within this band. A paired t-test was performed to test whether the bias was significant. The results were further interrogated via regression analysis to determine whether patient demographic (age, gender, side, BMI) had significant influence on the accuracy of the Visionaire jigs. Comparison data was obtained from the most recent 45 TKRs done in our hospital by the same surgeons using the same implants with conventional techniques. Data was obtained on patient demographics, surgical times and number of trays used.
Results Forty-five consecutive patients were recruited into the study; however, resection data was not available for the first four. Thus, 41 knees belonging to 40 patients were included in this study; 33 were females (one having both knees done during this study) and seven
454
Nankivell et al.
Fig. 1. Histograms of predicted minus actual resection thickness (difference in mm on x axis) for the 41 patient-specific cutting guide (PSCG) total knee replacement (TKRs): (a) distal femur medial resection, (b) distal femur lateral resection, (c) posterior femoral condyle medial resection, (d) posterior femoral condyle lateral resection, (e) proximal tibia medial resection, (f) proximal tibia lateral resection.
males. Their average age was 70.8 (±9.6) years, with an average BMI of 33.7 and a median American Society of Anesthesiologists (ASA) Score of 2. Seventeen operations were left sided and 24 were right. There were no significant differences between the PSCG and comparison groups with respect to patient demographics, skin-toskin time, tourniquet time and the number of trays used (Table 1). On further analysis, patient demographics (gender, age, side of operation and BMI) had no significant influence on the accuracy of the resections. The mean (± standard deviation) differences between predicted and actual resection thicknesses (‘resection error’) are shown in Table 2. The resection error of the distal femur (0.22 mm medial and 0.05 mm lateral) did not reach statistical significance. The resection error of the posterior femoral condyles (0.99 mm medial and
0.74 mm lateral) and the proximal tibia (0.71 mm medial and 0.55 mm lateral) did reach statistical significance. Histograms of these results have been presented in Figure 1.
Discussion This study shows that MRI-based PSCGs are accurate to within 1 mm on average. The other postulated benefits of PSCGs (reduced operating time, reduced tourniquet time and reduced number of trays) have however not been supported in this present study.
Accuracy Resections of the distal femur were within ±0.22 mm of predicted value. Thus, we are satisfied that the PSCGs cut where they are © 2014 Royal Australasian College of Surgeons
Accuracy of patient-specific cutting guides in TKR
Fig. 2. Line chart of number of trays used. The trend line shows that as the study progressed, fewer trays tended to be used. Note that no data are available for two cases (cases 11 and 38) as no record was made of the number of trays used.
planned. However, it is worth noting that greater variation was noted in the posterior femoral condyle resections, which are made through the standard cutting guide not the PSCG (however, the positioning of the standard cutting block is determined by PSCG). This implies the potential for loss of accuracy of the implanted femoral component, particularly with respect to rotation. This current study did not assess final implant positioning, which is currently being assessed by our team and forms the basis of a further study currently in progress.8 Furthermore, data was not collected on the number of knees requiring recuts for ‘soft tissue balancing’, and TKR stability, particularly flexion and extension stability, was not formally assessed in this paper.
Skin time and tourniquet time The consultant surgeons involved in this study performed the surgery using their own standard approach. Accordingly, some deflate the tourniquet before closing in order to achieve haemostasis, some leave the tourniquet inflated for the duration of the procedure. Furthermore, a tourniquet was not used in one case because of the presence of severe peripheral vascular disease. In another case, the tourniquet was deflated after 24 min (after soft tissue approach and bony resection, but before positioning of implants). There being no standardized approach to using a tourniquet in this study no firm conclusions about tourniquet time can be made. An average tourniquet time of 82 and skin-to-skin time of 84 min is comparable with our experience with conventional TKRs (Table 1).
Trays and cost-effectiveness Trays of instruments and implant trials all take time to handle and bear a cost associated with sterilization and packaging. Therefore, it has been proposed by others5 that if PSCGs can reduce the number of trays used, they will increase theatre (and sterilization department) efficiency and reduce costs to the health system. We did not collate patient theatre time or set-up time. In their study, Nunley et al. found no significant reduction in operating time but did find
© 2014 Royal Australasian College of Surgeons
455
theatre time reduced by 12 min per case. They suggested this reflected reduced set-up time (and therefore some reduced cost).6 In this study, we did not find any significant difference in skin time, tourniquet time or the number of trays used compared with conventional TKRs (Table 1). Therefore, claims of increased theatre (and health system) efficiency are not supported by our results. However, this study was undertaken early in the process of our surgeons’ use of PSCGs, and therefore, our theatre nursing staff had relative inexperience with the trays and set ups. The range in number of trays used (from two to seven) shows this variability. One case involved the opening of three wrong-sized trays necessitating the opening of three more and a further tray for patella resurfacing (thus a total of seven trays). Over the course of this study, less of trays were required (see the trend line in Figure 2), suggestive of a learning curve effect.
Conclusion It is important that our tools do what we think they do. This study measures the accuracy of PSCGs; however, this does not show evidence of the other purported benefits of PSCGs (e.g. increased efficiency). Furthermore, this paper has not assessed the accuracy of final prosthesis positioning as a result of the use of PSCGs. This is an area requiring further investigation. Finally, perhaps of more significance to patients is their satisfaction in terms of pain and function. More work needs to be done to determine if PSCGs lead to beneficial clinical outcomes (improved function and greater patient satisfaction) compared with conventional TKRs.
References 1. Lombardi AV Jr, Berend KR, Adams JB. Patient-specific approach in total knee arthroplasty. Orthopedics 2008; 31: 927–30. 2. Bali K, Walker P, Bruce W. Custom-fit total knee arthroplasty: our initial experience in 32 knees. J. Arthroplasty 2012; 27: 1149–54. 3. Bathis H, Perlick L, Tingart M, Luring C, Zurakowski D, Grifka J. Alignment in total knee arthroplasty. A comparison of computer-assisted surgery with the conventional technique. J. Bone Joint Surg. Br. 2004; 86: 682–7. 4. Howell SM, Kuznik K, Hull ML, Siston RA. Results of an initial experience with custom-fit positioning total knee arthroplasty in a series of 48 patients. Orthopedics 2008; 31: 857–63. 5. Noble JW Jr, Moore CA, Liu N. The value of patient-matched instrumentation in total knee arthroplasty. J. Arthroplasty 2012; 27: 153–5. 6. Nunley RM, Ellison BS, Ruh EL et al. Are patient-specific cutting blocks cost-effective for total knee arthroplasty? Clin. Orthop. Relat. Res. 2012; 470: 889–94. 7. Biant LC, Yeoh K, Walker PM, Bruce WJM, Walsh WR. The accuracy of bone resections made during computer navigated total knee replacement. Do we resect what the computer plans we resect? Knee 2008; 15: 238–41. 8. Pourgiezis N, Putalupattu S. Alignment and component position in patient-specific total knee arthroplasty: a prospective comparative study using Perth CT protocol (pending publication). 2014.
