ORTHOPAEDICS ANZJSurg.com
Component position alignment with patient-specific jigs in total knee arthroplasty Terence R. Moopanar,* Jeevaka E. Amaranath* and Rami M. Sorial† *Department of Orthopaedics, Nepean Public Hospital, Penrith, New South Wales, Australia and †Nepean Public Hospital/Nepean Private Hospital, University of Sydney, Penrith, New South Wales, Australia
Key words alignment in TKA, patient-specific jigs, total knee arthroplasty. Correspondence Dr Terence R. Moopanar, Department of Orthopaedics, Nepean Public Hospital, 86 Coonara Avenue, West Pennant Hills, NSW 2125, Australia. Email: [email protected] T. R. Moopanar MBBS; J. E. Amaranath MBBS; R. M. Sorial MBBS, FRACS, FAOrthoA. Accepted for publication 10 April 2014. doi: 10.1111/ans.12674
Abstract Background: The failure to restore mechanical alignment and appropriate rotational axis intraoperatively has been described as one of the most common causes of implant failure in total knee arthroplasty (TKA). Both conventional and computer-assisted TKA have their limitations. Patient-specific jigs (PSJ) offer a possible alternative method for TKA. The aim of this study was to investigate if the use of PSJ offers reproducible and accurate orientation of the components in TKA compared with conventional and computer-assisted surgery. Methods: We conducted a prospective case series looking at 261 consecutive patients undergoing TKA for osteoarthritis using the Signature Patient Specific System (Biomet, North Ryde, NSW, Australia). Each patient underwent a preoperative magnetic resonance imaging for planning. Using a computer software program, specialized femoral and tibial pin placement jigs were generated. Post-operative femoral and tibial component alignment was measured using computed tomography. Results: Of patients, 96.2% achieved femoral rotational alignment ±3 degrees of the transepicondylar axis. Tibial coronal alignment showed 92.7% of cases were 90 ± 3 degrees to the tibial medullary axis. Implant measurements of the posterior tibial slope demonstrated 76.6% of cases were within our accepted 0 to 7 degrees slope and 81.2% of patients had an overall mechanical axis within ±3 degrees of neutral. We also recorded femoral coronal alignment of the last 98 patients of our group and found that 99% were within 90 ± 3 degrees. Conclusion: PSJ for TKA shows good accuracy in alignment when compared with conventional TKA. However, improvements in the development of the tibial alignment cutting guides will aid in further increasing its overall accuracy and reproducibility.
Introduction Since its inception in the 1950s, total knee arthroplasty (TKA) has been regarded as a very successful orthopaedic procedure.1 Throughout this time, there have been dramatic advancements in knowledge of knee mechanics that have led to modifications of technique and design, which aid in making the prosthesis more durable and long lasting.2,3 Ultimately, such research aims to reduce the risk of implant failure. The failure to restore mechanical alignment and appropriate rotational axis intraoperatively, has been described as one of the most common causes of implant failure.4 Previous studies have clearly demonstrated the correlation between implant alignment and clinical outcome. Internal rotation of the femoral component is associated ANZ J Surg 84 (2014) 628–632
with patella–femoral complications including anterior knee pain, patella subluxation/dislocation and component failure.5,6 Excessive external rotation of the femoral component has been shown to cause an increased medial flexion gap and symptomatic flexion instability.7,8 Deviations in excess of the accepted 3 degrees of varus or valgus in the coronal mechanical axis have been shown to increase polyethylene load contributing to increased wear and decreased survival of the prosthesis.9–11 More recently, it has been shown that the margin of error in implant placement is narrow and implant failure is statistically appreciable when components are marginally malaligned.12 Conventional TKA relies on visual references and identifying bony landmarks as critical points for restoration of mechanical alignment and component placement. Studies have shown that the © 2014 Royal Australasian College of Surgeons
Component position alignment
accuracy and reproducibility of identifying these landmarks can be variable and will have an effect on overall alignment.13 Computer navigation appeared to present one answer to improving accuracy of implant placement intraoperatively. A number of studies have shown that computer navigation offers a better rate of accuracy and fewer outliers with regard to prosthetic implant placement than the use of standard instrumentation.14,15 Although this may be considered the current gold standard, the limitations of computer-navigated surgery include high set-up costs, pin site complications and increased operative time. A few case reports and small studies have recently advocated image based patient-specific instrumentation when performing TKA.16,17 These jigs are produced after a preoperative magnetic resonance imaging (MRI) of the limb is taken and anatomical landmarks for alignment are identified. Coronal, sagittal and rotational positioning of the prosthesis is then fashioned using a propriety software program with the surgeons input to restore mechanical alignment. Pin guides to position the femoral and tibial cutting blocks are then manufactured for the surgical procedure. We adopted the use of patient-specific jigs (PSJ) looking for a more accurate and reproducible prosthetic implant alignment, while reducing the limitations associated with computer navigation TKA mentioned earlier. The aim of this study was to investigate whether our use of a PSJ system offers reproducible and accurate orientation of the components in TKA. We also assessed the short-term clinical outcomes of patients undergoing TKA using PSJ. The results of our cohort group were then compared with published results for standard instrumentation and computer navigation.
Materials and methods We conducted a prospective case series looking at patients undergoing TKA by a single surgeon. The first 261 patients from June 2009 to December 2012 to have surgery using this method and then assessed by CT were included in this study. All patients underwent TKA using the SignatureTM patient-specific system (Biomet, North Ryde, NSW, Australia). using the cementless Vanguard knee replacement prosthesis (Biomet).Awritten informed consent was taken from every patient and ethics approval confirmed (Nepean Hospital, Kingswood, NSW, Australia) to review and report data. Preoperative workup included documentation of age, sex and range of motion. The majority of patients were imaged preoperatively by MRI (except for 10 patients who required CT imaging because of presence of pacemaker or claustrophobia). The MRI images were taken by a 1.5-T scanner (GE Medical Systems, Mansfield, QLD, Australia). A knee MRI scanning protocol consisting of four imaging sequences was followed. The images data file was then forwarded to the manufacturer for segmentation and the production of 3-D reconstructions to determine preoperative mechanical alignment, anatomical landmarks, implant sizes and ultimately implant placement. Using specialized computer software analysing anatomical landmarks and surgeon’s input (on alignment, rotation and any additional femoral resection based on preoperative flexion deformity), specialized femoral and tibial pin placement jigs were generated uniquely for each patient. Alignment was defined by the mechanical axis and rotation by the transepicondylar axis. © 2014 Royal Australasian College of Surgeons
629
TKA was carried out in the supine position with a tourniquet inflated to 300 mm Hg using a medial para-patella approach. All patients in the study underwent either a posterior cruciate ligament (PCL) retaining or PCL substituting TKA (Vanguard, Biomet). After meticulous soft tissue clearance the patient-specific guides, Signature Patient Guide System (Biomet), were placed directly on the bony and chondral surfaces for pin placement. Osteophytes were left intact as the PSJ accommodated for them as a means of improving accuracy of fit for each patient. Jig placement intraoperatively was compared and referenced to the MRI-generated model of the proximal tibia and distal femur in an effort to reproduce accurate placement outlined in the preoperative surgical plan. A removable drop rod allowed visual confirmation of coronal and sagittal positioning. Following pin placement, the standard tibial and femoral cutting blocks were then utilized over the pins and osteotomies performed. This is a measured resection technique followed with appropriate soft tissue balancing as needed. With the pins still in situ if the extension space is tight a further 2 mm can be resected from the distal femur using the standard cutting blocks. Following implantation of the components, wounds were closed in layers over a drain with surgical staples to the skin, which were removed on Day 7. After the first 6 weeks patients had a CT scan of the operated knee using a modified Perth CT protocol18 to determine the accuracy of implant placement. All scans were performed and read by the same senior radiographer. Measured parameters for the whole cohort included femoral component rotation, tibial component coronal alignment, tibial component posterior slope and overall mechanical limb alignment (centre of femoral head to centre of knee to centre of ankle). Femoral component coronal alignment was measured for the last 98 patients in this cohort. Statistical analysis was performed using Microsoft Excel and used the Excel ToolPak for basic data analysis including summary statistics and the generation of tables and figures. Student’s t-test was performed with mean and standard deviation recorded. We used P < 0.05 as threshold of significance.
Results The first consecutive 261 patients (159 women, 102 men) undergoing TKA using PSJ and assessed by CT are reported on in our study. The average age was 66.2 ± 8.6 years, with 45.9% being left. Preoperative alignment measures showed that 22% of knees were in valgus, 3% in neutral and 74% in varus as referenced to the mechanical axis. The average preoperative deformity was 3.96 degrees of varus. With respect to rotation of the femoral component in the axial plane, the transepicondyllar axis (TEA) was used as the reference line for our CT measurements of component alignment. The angular rotation of the femoral component with respect to the TEA to be on average 0.8 ± 1.1 of external rotation (range from 6.4 degrees internal rotation–4.6 degrees external rotation). Femoral components in 96.2% of patients were within 3 degrees of the TEA reference line. Internal rotation of the femoral component occurred in less than 2% of our patient population (Fig. 1). Femoral component coronal alignment to mechanical axis was also recorded for the last 98 patients of the cohort and 99% of patients had alignment measures of 90 ± 3 degrees.
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Moopanar et al.
Femoral Rotaon
8
Posterior Tibial Slope 16 14 12
4 2 0
0
50
100
150
20 00 200
250
300
–2 –4
Posterior Slope (Degrees)
Femoral Rotaon (Degrees)
6
–6
2 0 –2
0
50
100
150
200
250
300
–4
Paent Number
–10 Fig. 3. The scatter plot of the component posterior tibial slope plotted against patient number shows the distribution of tibial slopes within the range of 0 to 7 degrees (shaded area). The negative values represent an anterior slope.
Mechanical Axis
96 94 92 0
50 0
100 1 0 10
150 150 15
200 2 0 20
250 25 50
300 30
86 84 82 Paent Number
Fig. 2. The tibial component coronal alignment plotted against the first 261 patients. The shaded area depicts the acceptable margin of alignment (±3 degrees).
For the coronal alignment of the tibial component, the desired implant placement is at 90 degrees to the longitudinal axis of the tibia (neutral). On average, tibial component placement was measured at 90.7 degrees (range 86–95 degrees). Tibial components in 92.7% of patients were aligned within 3 degrees of the neutral position. Varus alignment outliers (93 degrees) was seen in 5.8% (Fig. 2). The posterior slope of the tibial component was placed at 4.9 ± 2.7 degrees (range anterior slope 3.6–11 degrees posterior slope) on average. 76.6% of patients had tibial components aligned within the range of 0 to 7 degrees. Anterior slope outliers were noted in less than 2% of patients (Fig. 3).
10 9 8 7 6 5 4 3 2 1 0 –1 0 –2 –3 –4 –5 –6 –7 –8 –9 –10
Mechanical Axis (Degrees)
98 Tibial Alignment (degrees)
4
Tibial Coronal Alignment
100
80
6
–8
Paent number
Fig. 1. The rotation of the femoral component in the axial plain for the first 261 patients. The shaded area depicts the acceptable margin of alignment (±3 degrees). The negative values represent internal rotation of the component. The positive values represent external rotation of the component.
88
8
–6
–8
90
10
50 50
100
150
200
250 0
30 300
Paent Number
Fig. 4. The component mechanical axis plotted for each patient. The negative values represent varus alignment, positive values represent valgus alignment and neutral alignment is at 0 degrees.
Finally, the overall mechanical axis averaged at 1.1 ± 2.4 degrees (valgus) (range 6 degrees varus to 7 degrees valgus) postoperatively. 81.2% of patients to have a mechanical axis within 3 degrees of neutral alignment. Of the remaining outliers 14.5% had >3 degrees of valgus alignment and 4.3% had >3 degrees of varus alignment (Fig. 4).
Discussion The success of a total knee replacement can depend primarily on the restoration of the patient’s mechanical and rotational alignment.2,5 This paper has shown that signature PSJ produce very accurate © 2014 Royal Australasian College of Surgeons
Component position alignment
results for femoral component alignment as measured on coronal (99%) and axial (96.2%) CT scans of the femoral prosthesis. But tibial component alignment does not match this same high level of accuracy on coronal (91.7%) and sagittal (76.6%) CT scans of the tibial prosthesis. The additive result is an overall mechanical alignment for the group that has 81.2% in the neutral ±3 degree range. The authors believe this can only be improved further by modifying the tibial pin guides to ensure more enhanced capture of the patient’s tibial articular and anterior cortical surfaces. Although not specifically measured, it was noted throughout the series that the femoral guides always had an accurate and secure fit to the femoral surface, but the tibial guides occasionally had a few degrees of play that required the drop rod attachment to double check alignment along the anterior tibial anterior crest visually. Improving the security of this fit by increasing anterior cortical contact with the tibia and further posterior articular extension of the guide is needed to improve tibial prosthetic alignment further. Restoration of the femoral rotational axis is universally recognized as important in reducing implant failure.5–8 We found that 96.2% of all our cases were ±3 degrees of neutral. The majority of these were externally rotated. The accuracy of femoral rotation alignment is higher than conventional TKR and within range or better than computer-assisted TKR.19 These results demonstrate that PSJ instrumentation can reduce the variability in femoral rotational alignment seen previously with conventional and navigated TKAs20 principally because it utilizes an image-based selection of the anatomical landmarks rather than relying on surgeon selection of these landmarks intraoperatively. The tibial component placement in the coronal plane showed that 92.7% of cases reached an alignment of 90 ± 3 degrees. Furthermore, the outliers were mainly in valgus and not varus. This is significant as studies show that a tibial component alignment of greater than 3 degrees of varus is associated with implant failure and revision surgery.21 A large retrospective study by Ritter et al.10 reviewing 6070 TKR found a high failure rate (8.7%) was associated with patients that had varus (
Component position alignment with patient-specific jigs in total knee arthroplasty Terence R. Moopanar,* Jeevaka E. Amaranath* and Rami M. Sorial† *Department of Orthopaedics, Nepean Public Hospital, Penrith, New South Wales, Australia and †Nepean Public Hospital/Nepean Private Hospital, University of Sydney, Penrith, New South Wales, Australia
Key words alignment in TKA, patient-specific jigs, total knee arthroplasty. Correspondence Dr Terence R. Moopanar, Department of Orthopaedics, Nepean Public Hospital, 86 Coonara Avenue, West Pennant Hills, NSW 2125, Australia. Email: [email protected] T. R. Moopanar MBBS; J. E. Amaranath MBBS; R. M. Sorial MBBS, FRACS, FAOrthoA. Accepted for publication 10 April 2014. doi: 10.1111/ans.12674
Abstract Background: The failure to restore mechanical alignment and appropriate rotational axis intraoperatively has been described as one of the most common causes of implant failure in total knee arthroplasty (TKA). Both conventional and computer-assisted TKA have their limitations. Patient-specific jigs (PSJ) offer a possible alternative method for TKA. The aim of this study was to investigate if the use of PSJ offers reproducible and accurate orientation of the components in TKA compared with conventional and computer-assisted surgery. Methods: We conducted a prospective case series looking at 261 consecutive patients undergoing TKA for osteoarthritis using the Signature Patient Specific System (Biomet, North Ryde, NSW, Australia). Each patient underwent a preoperative magnetic resonance imaging for planning. Using a computer software program, specialized femoral and tibial pin placement jigs were generated. Post-operative femoral and tibial component alignment was measured using computed tomography. Results: Of patients, 96.2% achieved femoral rotational alignment ±3 degrees of the transepicondylar axis. Tibial coronal alignment showed 92.7% of cases were 90 ± 3 degrees to the tibial medullary axis. Implant measurements of the posterior tibial slope demonstrated 76.6% of cases were within our accepted 0 to 7 degrees slope and 81.2% of patients had an overall mechanical axis within ±3 degrees of neutral. We also recorded femoral coronal alignment of the last 98 patients of our group and found that 99% were within 90 ± 3 degrees. Conclusion: PSJ for TKA shows good accuracy in alignment when compared with conventional TKA. However, improvements in the development of the tibial alignment cutting guides will aid in further increasing its overall accuracy and reproducibility.
Introduction Since its inception in the 1950s, total knee arthroplasty (TKA) has been regarded as a very successful orthopaedic procedure.1 Throughout this time, there have been dramatic advancements in knowledge of knee mechanics that have led to modifications of technique and design, which aid in making the prosthesis more durable and long lasting.2,3 Ultimately, such research aims to reduce the risk of implant failure. The failure to restore mechanical alignment and appropriate rotational axis intraoperatively, has been described as one of the most common causes of implant failure.4 Previous studies have clearly demonstrated the correlation between implant alignment and clinical outcome. Internal rotation of the femoral component is associated ANZ J Surg 84 (2014) 628–632
with patella–femoral complications including anterior knee pain, patella subluxation/dislocation and component failure.5,6 Excessive external rotation of the femoral component has been shown to cause an increased medial flexion gap and symptomatic flexion instability.7,8 Deviations in excess of the accepted 3 degrees of varus or valgus in the coronal mechanical axis have been shown to increase polyethylene load contributing to increased wear and decreased survival of the prosthesis.9–11 More recently, it has been shown that the margin of error in implant placement is narrow and implant failure is statistically appreciable when components are marginally malaligned.12 Conventional TKA relies on visual references and identifying bony landmarks as critical points for restoration of mechanical alignment and component placement. Studies have shown that the © 2014 Royal Australasian College of Surgeons
Component position alignment
accuracy and reproducibility of identifying these landmarks can be variable and will have an effect on overall alignment.13 Computer navigation appeared to present one answer to improving accuracy of implant placement intraoperatively. A number of studies have shown that computer navigation offers a better rate of accuracy and fewer outliers with regard to prosthetic implant placement than the use of standard instrumentation.14,15 Although this may be considered the current gold standard, the limitations of computer-navigated surgery include high set-up costs, pin site complications and increased operative time. A few case reports and small studies have recently advocated image based patient-specific instrumentation when performing TKA.16,17 These jigs are produced after a preoperative magnetic resonance imaging (MRI) of the limb is taken and anatomical landmarks for alignment are identified. Coronal, sagittal and rotational positioning of the prosthesis is then fashioned using a propriety software program with the surgeons input to restore mechanical alignment. Pin guides to position the femoral and tibial cutting blocks are then manufactured for the surgical procedure. We adopted the use of patient-specific jigs (PSJ) looking for a more accurate and reproducible prosthetic implant alignment, while reducing the limitations associated with computer navigation TKA mentioned earlier. The aim of this study was to investigate whether our use of a PSJ system offers reproducible and accurate orientation of the components in TKA. We also assessed the short-term clinical outcomes of patients undergoing TKA using PSJ. The results of our cohort group were then compared with published results for standard instrumentation and computer navigation.
Materials and methods We conducted a prospective case series looking at patients undergoing TKA by a single surgeon. The first 261 patients from June 2009 to December 2012 to have surgery using this method and then assessed by CT were included in this study. All patients underwent TKA using the SignatureTM patient-specific system (Biomet, North Ryde, NSW, Australia). using the cementless Vanguard knee replacement prosthesis (Biomet).Awritten informed consent was taken from every patient and ethics approval confirmed (Nepean Hospital, Kingswood, NSW, Australia) to review and report data. Preoperative workup included documentation of age, sex and range of motion. The majority of patients were imaged preoperatively by MRI (except for 10 patients who required CT imaging because of presence of pacemaker or claustrophobia). The MRI images were taken by a 1.5-T scanner (GE Medical Systems, Mansfield, QLD, Australia). A knee MRI scanning protocol consisting of four imaging sequences was followed. The images data file was then forwarded to the manufacturer for segmentation and the production of 3-D reconstructions to determine preoperative mechanical alignment, anatomical landmarks, implant sizes and ultimately implant placement. Using specialized computer software analysing anatomical landmarks and surgeon’s input (on alignment, rotation and any additional femoral resection based on preoperative flexion deformity), specialized femoral and tibial pin placement jigs were generated uniquely for each patient. Alignment was defined by the mechanical axis and rotation by the transepicondylar axis. © 2014 Royal Australasian College of Surgeons
629
TKA was carried out in the supine position with a tourniquet inflated to 300 mm Hg using a medial para-patella approach. All patients in the study underwent either a posterior cruciate ligament (PCL) retaining or PCL substituting TKA (Vanguard, Biomet). After meticulous soft tissue clearance the patient-specific guides, Signature Patient Guide System (Biomet), were placed directly on the bony and chondral surfaces for pin placement. Osteophytes were left intact as the PSJ accommodated for them as a means of improving accuracy of fit for each patient. Jig placement intraoperatively was compared and referenced to the MRI-generated model of the proximal tibia and distal femur in an effort to reproduce accurate placement outlined in the preoperative surgical plan. A removable drop rod allowed visual confirmation of coronal and sagittal positioning. Following pin placement, the standard tibial and femoral cutting blocks were then utilized over the pins and osteotomies performed. This is a measured resection technique followed with appropriate soft tissue balancing as needed. With the pins still in situ if the extension space is tight a further 2 mm can be resected from the distal femur using the standard cutting blocks. Following implantation of the components, wounds were closed in layers over a drain with surgical staples to the skin, which were removed on Day 7. After the first 6 weeks patients had a CT scan of the operated knee using a modified Perth CT protocol18 to determine the accuracy of implant placement. All scans were performed and read by the same senior radiographer. Measured parameters for the whole cohort included femoral component rotation, tibial component coronal alignment, tibial component posterior slope and overall mechanical limb alignment (centre of femoral head to centre of knee to centre of ankle). Femoral component coronal alignment was measured for the last 98 patients in this cohort. Statistical analysis was performed using Microsoft Excel and used the Excel ToolPak for basic data analysis including summary statistics and the generation of tables and figures. Student’s t-test was performed with mean and standard deviation recorded. We used P < 0.05 as threshold of significance.
Results The first consecutive 261 patients (159 women, 102 men) undergoing TKA using PSJ and assessed by CT are reported on in our study. The average age was 66.2 ± 8.6 years, with 45.9% being left. Preoperative alignment measures showed that 22% of knees were in valgus, 3% in neutral and 74% in varus as referenced to the mechanical axis. The average preoperative deformity was 3.96 degrees of varus. With respect to rotation of the femoral component in the axial plane, the transepicondyllar axis (TEA) was used as the reference line for our CT measurements of component alignment. The angular rotation of the femoral component with respect to the TEA to be on average 0.8 ± 1.1 of external rotation (range from 6.4 degrees internal rotation–4.6 degrees external rotation). Femoral components in 96.2% of patients were within 3 degrees of the TEA reference line. Internal rotation of the femoral component occurred in less than 2% of our patient population (Fig. 1). Femoral component coronal alignment to mechanical axis was also recorded for the last 98 patients of the cohort and 99% of patients had alignment measures of 90 ± 3 degrees.
630
Moopanar et al.
Femoral Rotaon
8
Posterior Tibial Slope 16 14 12
4 2 0
0
50
100
150
20 00 200
250
300
–2 –4
Posterior Slope (Degrees)
Femoral Rotaon (Degrees)
6
–6
2 0 –2
0
50
100
150
200
250
300
–4
Paent Number
–10 Fig. 3. The scatter plot of the component posterior tibial slope plotted against patient number shows the distribution of tibial slopes within the range of 0 to 7 degrees (shaded area). The negative values represent an anterior slope.
Mechanical Axis
96 94 92 0
50 0
100 1 0 10
150 150 15
200 2 0 20
250 25 50
300 30
86 84 82 Paent Number
Fig. 2. The tibial component coronal alignment plotted against the first 261 patients. The shaded area depicts the acceptable margin of alignment (±3 degrees).
For the coronal alignment of the tibial component, the desired implant placement is at 90 degrees to the longitudinal axis of the tibia (neutral). On average, tibial component placement was measured at 90.7 degrees (range 86–95 degrees). Tibial components in 92.7% of patients were aligned within 3 degrees of the neutral position. Varus alignment outliers (93 degrees) was seen in 5.8% (Fig. 2). The posterior slope of the tibial component was placed at 4.9 ± 2.7 degrees (range anterior slope 3.6–11 degrees posterior slope) on average. 76.6% of patients had tibial components aligned within the range of 0 to 7 degrees. Anterior slope outliers were noted in less than 2% of patients (Fig. 3).
10 9 8 7 6 5 4 3 2 1 0 –1 0 –2 –3 –4 –5 –6 –7 –8 –9 –10
Mechanical Axis (Degrees)
98 Tibial Alignment (degrees)
4
Tibial Coronal Alignment
100
80
6
–8
Paent number
Fig. 1. The rotation of the femoral component in the axial plain for the first 261 patients. The shaded area depicts the acceptable margin of alignment (±3 degrees). The negative values represent internal rotation of the component. The positive values represent external rotation of the component.
88
8
–6
–8
90
10
50 50
100
150
200
250 0
30 300
Paent Number
Fig. 4. The component mechanical axis plotted for each patient. The negative values represent varus alignment, positive values represent valgus alignment and neutral alignment is at 0 degrees.
Finally, the overall mechanical axis averaged at 1.1 ± 2.4 degrees (valgus) (range 6 degrees varus to 7 degrees valgus) postoperatively. 81.2% of patients to have a mechanical axis within 3 degrees of neutral alignment. Of the remaining outliers 14.5% had >3 degrees of valgus alignment and 4.3% had >3 degrees of varus alignment (Fig. 4).
Discussion The success of a total knee replacement can depend primarily on the restoration of the patient’s mechanical and rotational alignment.2,5 This paper has shown that signature PSJ produce very accurate © 2014 Royal Australasian College of Surgeons
Component position alignment
results for femoral component alignment as measured on coronal (99%) and axial (96.2%) CT scans of the femoral prosthesis. But tibial component alignment does not match this same high level of accuracy on coronal (91.7%) and sagittal (76.6%) CT scans of the tibial prosthesis. The additive result is an overall mechanical alignment for the group that has 81.2% in the neutral ±3 degree range. The authors believe this can only be improved further by modifying the tibial pin guides to ensure more enhanced capture of the patient’s tibial articular and anterior cortical surfaces. Although not specifically measured, it was noted throughout the series that the femoral guides always had an accurate and secure fit to the femoral surface, but the tibial guides occasionally had a few degrees of play that required the drop rod attachment to double check alignment along the anterior tibial anterior crest visually. Improving the security of this fit by increasing anterior cortical contact with the tibia and further posterior articular extension of the guide is needed to improve tibial prosthetic alignment further. Restoration of the femoral rotational axis is universally recognized as important in reducing implant failure.5–8 We found that 96.2% of all our cases were ±3 degrees of neutral. The majority of these were externally rotated. The accuracy of femoral rotation alignment is higher than conventional TKR and within range or better than computer-assisted TKR.19 These results demonstrate that PSJ instrumentation can reduce the variability in femoral rotational alignment seen previously with conventional and navigated TKAs20 principally because it utilizes an image-based selection of the anatomical landmarks rather than relying on surgeon selection of these landmarks intraoperatively. The tibial component placement in the coronal plane showed that 92.7% of cases reached an alignment of 90 ± 3 degrees. Furthermore, the outliers were mainly in valgus and not varus. This is significant as studies show that a tibial component alignment of greater than 3 degrees of varus is associated with implant failure and revision surgery.21 A large retrospective study by Ritter et al.10 reviewing 6070 TKR found a high failure rate (8.7%) was associated with patients that had varus (
