MANAGEMENT FACTORIALS IN TOTAL KNEE ARTHROPLASTY
Soft-tissue and alignment correction THE USE OF SMART TRIALS IN TOTAL KNEE REPLACEMENT
K. A. Gustke From Florida Orthopaedic Institute, Tampa, Florida, United States
Total knee replacement (TKR) smart tibial trials have load-bearing sensors which will show quantitative compartment pressure values and femoral-tibial tracking patterns. Without smart trials, surgeons rely on feel and visual estimation of imbalance to determine if the knee is optimally balanced. Corrective soft-tissue releases are performed with minimal feedback as to what and how much should be released. The smart tibial trials demonstrate graphically where and how much imbalance is present, so that incremental releases can be performed. The smart tibial trials now also incorporate accelerometers which demonstrate the axial alignment. This now allows the surgeon the option to perform a slight recut of the tibia or femur to provide soft-tissue balance without performing soft-tissue releases. Using a smart tibial trial to assist with soft-tissue releases or bone re-cuts, improved patient outcomes have been demonstrated at one year in a multicentre study of 135 patients (135 knees). Cite this article: Bone Joint J 2014;96-B(11 Suppl A):78–83.
K. A. Gustke, MD, Adjunct Clinical Professor of Orthopaedic Surgery University of South Florida, Tampa, Florida, and Florida Orthopaedic Institute, 13020 North Telecom Parkway, Temple Terrace, Florida, USA. Correspondence should be sent to Dr K. A. Gustke; e-mail: [email protected] ©2014 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.96B11. 34339 $2.00 Bone Joint J 2014;(11 Suppl A):78–83.
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Studies spanning 15 years have reported excellent 85% to 97% survivorship with total knee replacement (TKR).1 However, excellent survivorship does not equate to excellent patient reported outcomes and for example, Noble et al2 reported that 14% of their patients were dissatisfied with their outcome. The majority of these patients had problems with kneeling, squatting, gardening, lateral movements, and carrying loads. There is also a difference in the patients subjective assessment of outcome and the surgeons objective assessment.3 Dickstein et al4 reported that a third of TKR patients were dissatisfied, even though the surgeons felt that their results were excellent. Most of the patients who report lower outcome scores do so because their expectations are not being fulfilled by TKR.5 These low patient satisfactions are often puzzling. Modern TKR instrumentation can produce very accurate bone cuts and alignment. However, better alignment does not equate to better short-term function and outcomes.6 Radiological and physical examination of many of these less satisfied patients fail to reveal obvious correctable solutions. Our assumption is that many of these patients may be experiencing subtle soft-tissue imbalance and maltracking of the components that we have difficulty in assessing either intraor post-operatively. Surgeons are able to easily recognise knees that have gross instability and
both the patient and surgeon are in agreement as to why a poorer outcome has occurred. However, many more patients may have less detectable instability which the surgeon is not able to appreciate and the patient is usually unable to describe accurately. Current techniques for soft-tissue balancing rely on subjective feel and visual evaluation of the anteroposterior position of the femur on the tibia and medial and lateral gap assessment while varus and valgus stresses are applied. This technique is not very accurate. It is essentially an art with success related to the surgeons’ experience. In order to properly visualise the gaps, the surgeon needs to be able to see inside the joint. If the patella is everted or subluxated laterally, increased pressures are placed across the lateral compartment. When the knee is in flexion, without closure of the medial retinaculum, the posteromedial capsule is looser resulting in lower medial compartment pressures and a more posterior medial femoral contact point. The ‘smart tibial trial’ can eliminate the effect of inexperience on the judgment of amount of gap balance. It also allows the patella to be reduced and the medial retinaculum to be temporarily closed during assessment of component tracking and balance. We have used the OrthoSensor Knee Balancer smart tibial trial (OrthoSensor Inc., Dania Beach, Florida) to assess total knee balance in a series CCJR SUPPLEMENT TO THE BONE & JOINT JOURNAL
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Fig. 1
Fig. 2
Photograph showing the Verasense tibia liner with wireless microprocessors sensors to measure contact force and location.
Photograph showing the OrthoSensor Link station monitor with wireless connection to sensors
of patients (Fig. 1). The device has miniaturised integrated circuits and micro processers that are similar to those used in cell phones and it communicates to a computer via a wireless link (Fig. 2). The computer has a graphical interface which shows the pressure in pounds per square inch (psi) that are present and the location of maximal contact between the medial and lateral femoral condyles (Fig. 3). The pressure and component tracking can be assessed while taking the knee through a range of movement. These smart trials have the shape of standard implant trial devices, so it can be designed for use in different total knee systems. Currently, four major total knee systems make use of this product. They are single use, disposable, and fairly inexpensive. They are currently priced at US$ 459. They also allow the surgeon to use their preferred work flow, using classical gap balancing or measured resection for the bone cuts. The first generation of the OrthoSensor smart trial did not provide any alignment information. Standard instrumentation or computer navigation was relied on to obtain accurate component alignment. The OrthoSensor VERASENSE smart trial now incorporates accelerometers which show tibial axial alignment and overall hip-to-ankle mechanical axial alignment (Fig. 3) which is essential in making a decision on whether to perform a soft-tissue release. Knowing the alignment also gives the surgeon the option to slightly recut the distal femur or tibia avoiding having to perform a soft-tissue release.
The purpose of the multicentre study was to better define balance based on quantitative measurements and to determine if patients with quantifiably better balanced knee joints, achieved with the use of sensors, exhibited improved clinical outcomes. Patient pre- and post-operative anatomical alignment was obtained by measuring 18 inch standing radiographs of the knee. Clinical assessments and patient satisfaction was measured via American Knee Society Score,7 and Western Ontario and McMasters Universities Osteoarthritis Index (WOMAC) scores.8 A new Activity levels scoring system based on a 6 level 100 point scale was developed for the multicentre study in order to statistically quantify and better distinguish typical activities of a post-total knee replacement patient. The patients were asked to choose a category that best described their activity level from ‘bedridden’, ‘sedentary’, ‘semi-sedentary’, ‘light labour’, ‘moderate labour’, and ‘heavy labour’. They were provided with examples of what activities they would be expected to do in each category. Bedridden patients were either bedridden or confined to a wheelchair. A sedentary patient would have minimal ambulation or activity. Patients who can perform light house cleaning, white collar office work, or benchtype work would be considered semi-sedentary. A patient would be in the light labour category if they could perform heavy cleaning, assembly line work, or do light sports. ‘Moderate labour’ would consist of being able to lift up to 50 pounds (23 kg) and participate in moderate sports. Patients who could perform vigorous sports or lift 50 to 100 pounds (23 kg to 45 kg) were considered capable of ‘heavy labour’. Each descriptive category was designated with a numeric representation, at 20-point intervals: bedridden = 0, sedentary = 20, semi-sedentary = 40, light labour = 60, moderate labour = 80, and heavy labour = 100.
Our clinical experience with the balancing device A three-year prospective multicentre IRB monitored study was initiated in February 2012 for TKRs performed with the smart tibial trial using the Triathlon total knee system (Stryker, Mahwah, New Jersey). There were 135 knees in 135 patients with one year follow-up data. There were eight participating centres. VOL. 96-B, No. 11, NOVEMBER 2014
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Fig. 3b
Fig. 3a
Fig. 3c Graphic interfaces demonstrating a) compartment pressures and alignment, b) tight medial soft tissues causing elevated medial compartment pressures. Compartment pressures are more equalised after a medial soft-tissue release, and c) the tracking pattern of maximum contact pressure points as the knee is taken through a range of movement.
Use of the smart tibial trial was relatively new to all surgeons in this series and only one of the eight surgeons had previously used the Triathlon total knee system (Stryker) extensively before beginning the multicentre study. All surgeons were asked to use the device to assist in balancing the knees to their satisfaction. All knees were considered adequately clinically balanced by the surgeons at the end of the case based on their experience in assessing how the knees felt under varus/valgus and antero/posterior stress. When six month data was analysed on the initial study patients, it became evident that knees that had more equally balanced compartment pressures as determined by the smart trial were demonstrating better early outcomes. A sub-study of the initial multicentre study was initiated to determine if knees that had compartments more quantitatively balanced had better outcomes. Less than 15 PSI (103421 pascal) differences between the medial and lateral compartment pressures were arbitrarily chosen as the definition of satisfactory ‘balance’. A subsequent cadaver study by Walker, Meere and Bell9 validated that a 15 PSI (103421 pascal) difference correlates to optimal clinical balance. Using this definition, 18 (13%) of the 135 knees were ‘unbalanced.’ There was no difference in the ‘balanced’ or ‘unbalanced’ knees in age, sex, BMI, pre-operative alignment or range of movement. There
was enough statistical power for a comparison of the two groups (power 0.81). Most of the ‘unbalanced’ TKRs were performed in the early part of this series, suggesting the effect of a learning curve associated with new technology. The six-month results of the multicentre study have been reported and showed that satisfactorily ‘balanced’ knees according to our criteria had better patient reported outcomes and better activity scores.
The surgical technique when using the balancing device The femoral and tibial bone cuts are made as per the surgeon’s usual technique. The tibial components are inserted with the thickest standard tibial insert to eliminate the gap in the tighter medial or lateral compartment. With the knee in full extension, the VERASENSE smart trial is placed on the tibial crest at the level that is closest to being parallel to the tibial coronal plane to establish the tibial reference. The VERASENSE device is then placed into the knee using shims if necessary to increase the insert thickness. With the knee in full extension, the approximate anteroposterior slope of the tibia is graphically shown. With the knee near full extension and in perceived neutral rotation from the surgeons perspective, the tibial tray is rotated and CCJR SUPPLEMENT TO THE BONE & JOINT JOURNAL
SOFT-TISSUE AND ALIGNMENT CORRECTION
Fig. 4a
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Fig. 4b
Graphic interfaces demonstrating a) high lateral compartment pressures near full extension (note 2.5° varus mechanical alignment) and b) compartment pressures more equalised after 3° valgus recut of the distal femur.
fixed so that the medial and lateral femoral tibial contact points have similar distances to the anterior tibial cortex. The contact point rotation value is registered. With the heel planted, the knee is flexed until the flexion dial on the screen indicates that the knee is in an optimal degree of flexion for the accelerometer. The hip is abducted and adducted until the accelerometer determines the varus/ valgus tilt of the tibial component. The leg is then placed in extension and elevated until the mechanical axis indicates a good position. While applying axial pressure on the foot, the hip is rotated until the hip/knee/ankle mechanical axis is determined. If the tibial alignment or mechanical axis is not within the surgeon’s desired acceptable range, the tibia or distal femur is recut. With the medial retinaculum held closed with two or three towel clips, the knee is taken through a range of movement while viewing the compartment pressures. In order to avoid any abnormal varus or valgus force, the distal thigh is supported with one hand and the heel is supported with the other. As experience was gained by the multicentre surgeons using the smart trial, the Triathlon Knee patients appear to have best visual stability when compartment pressures are between 20 and 30 PSI (137895 to 206843 pascal). If both compartment pressures are much lower than this, the knees appeared to be too loose. Having low compartment pressures will also increase the chance that inadvertently applied varus or valgus, while taking the knee through range of movement, will affect the compartment pressures. A thicker shim is then placed under the sensor trial. If compartment pressures are within 15 PSI (103421 pascal) of each other, the knee is deemed adequately balanced. Range of movement can be also performed with the tracking function on to confirm proper component kinematics. If compartment pressures are more than 15 PSI (103421 pascal) different a decision is made as to whether a bone VOL. 96-B, No. 11, NOVEMBER 2014
recut or soft-tissue release should be performed. If a bone recut is chosen, a recut of tibia is done if the tight compartment is tight in flexion and extension. If the compartment is tight only in extension, a distal femur recut is performed. If the compartment is tight only in flexion, a change of femoral rotation is performed. If the posterior cruciate ligament is too tight causing excessive medial and lateral roll-back in flexion, the tibia can be recut with more slope. A minimal recut of 2 mm or 2° can usually change a knee from an ‘unbalanced’ to a ‘balanced’ knee.8 If a soft-tissue release is preferred, the sensor loads will change sequentially as more releasing is performed. This facilitates the use of a pie crusting release technique with an 18 gauge spinal needle or a #11 knife blade. A knee tighter in extension than flexion would warrant the release to be in the more posterior aspect of the soft-tissue sleeve. If an outside-in release is performed, the retinaculum can be held closed with towel clips while the releases are performed while visualising load decreases on the monitor. After several piercings of the soft tissue, the knee is cycled through the range of movement to allow the tissues to stretch. If an inside-out release is performed, it needs to be gradual. The retinaculum is intermittently closed, and any change in forces is noted while the knee is ranged. Figure 3a is from a case with tight medial soft tissues. Figure 3b graphically shows knee balance after a piecrusting release was performed on the tight medial soft tissues. Figure 3c demonstrates the preferred component maximum contact point tracking pattern. Figure 4a shows a case demonstrating a tight lateral compartment mainly near full extension. The overall alignment of this knee is 2.5° of varus. Rather than performing a release of the iliotibial band, a 3° valgus recut of the distal femur was performed. Figure 4b shows balanced compartments and an accepted 1.7° overall valgus alignment.
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Clinical update One year data results from 135 knees in 135 patients in the multicentre study series has been published.10 The quantitatively ‘balanced’ knees continue to show statistical improvement over the six month data.11 The mean total American Knee Society Score of the ‘balanced’ group is 23.3 points higher than the ‘unbalanced’ group (p < 0.001); 179 + 17.2 and 156 + 23.4 for the ‘balanced’ and ‘unbalanced’ patients. The mean Knee Society pain score was 96.4 for the ‘balanced’ group and 87.8 for the ‘unbalanced’ group (p < 0.001). The mean Knee Society function score was 82.4 for the ‘balanced group’ and 68.3 for the ‘unbalanced’ group (p = 0.022). Mean pre-operative anatomical knee alignment was 4.9° of valgus for the ‘balanced’ group and 5.1° of valgus for the ‘unbalanced’ group. Mean postoperative anatomical knee alignment was 4.52° of valgus for the ‘balanced’ group and 4.22° of valgus for the ‘unbalanced’ group. The ‘balanced’ group had an 8 point higher improvement in WOMAC scores than the ‘unbalanced’ group (10 + 11.8 and 18 + 17 for ‘balanced’ and ‘unbalanced’ patients, p = 0.085). The ‘balanced’ group mean activity score was 68.6, which corresponds with light to moderate labour categories. The ‘unbalanced’ group mean activity score was 46.7, which corresponds to a lower semisedentary category. The difference between the mean activity scores was statistically significant (p = 0.015). The data that was most compelling, was that the Knee Society pain and functional scores, WOMAC scores, and activity levels for the ‘unbalanced’ knees at one year had still not reached what the ‘balanced’ knees had achieved at six months (combined Knee Society pain and functional score of 172.4, WOMAC score of 14.5, and activity score of 40.5). Discussion Despite improvements in total knee implant design and surgical technique, a significant percentage of TKR patients report inferior outcomes.1-5 With the assumption that softtissue imbalance may be responsible for some of the inferior outcomes and that surgeons have difficulty with accurately balancing TKRs with conventional surgical techniques, smart tibial trials have been designed. Use of smart tibial trials has provided the ability to quantitatively measure the anteroposterior and mediolateral balance of a TKR intraoperatively with the medial retinaculum closed. The sensors demonstrate imbalances through overly tight soft tissues. They will also demonstrate a malrotated tibial or femoral component. Soft-tissue releases can be performed sequentially while visualising their effect on balancing the compartment pressures. The latest generation sensors have accelerometers to visualise the tibial component and overall axial alignment. If an imbalance is present and a minimal bone recut will still keep alignment within acceptable ranges, soft-tissue releases can be avoided. The results reported from use of a smart tibial trial, with the ability to measure knee compartment pressures and alignment, are unique to this device.
The limitations of both the six-month and one-year reported studies are that the number of ‘unbalanced’ knees is relatively small.10,11 The majority of knees that were in the ‘unbalanced’ group were performed early in the study, during the learning curve for the surgeons using a new device and total knee system. As experience and data results became known to the surgeon group, fewer knees were left with a greater than 15 PSI (103421 pascal) difference between the two compartments. Another limitation is that the surgeons were not blinded as to which patients had ‘unbalanced’ or ‘balanced’ knees. Even though the surgeons knew at the time of surgery what ultimate differences in compartment pressures were present, they were still satisfied that the knees were clinically balanced and the knowledge of which group the patient was in was not readily available at the time of follow-up assessments. The outcome data for the most part was independent of surgeon bias and the patients did not know if they were in the ‘unbalanced’ or ‘balanced group’. Another limitation of the study is that there is no control group of total knee replacements without using the sensor performed by the same group of surgeons. Improved outcomes scores could be enhanced by the fact that patients knew they were having a surgery with a technologically advanced device. However, the patients did not know if they were in the ‘balanced’ or ‘unbalanced’ groups. Thus the placebo effect would be expected to be present in both groups. One-year reported results show that knees that are better balanced using a smart tibial trial have better pain, functional, and activity scores.10 A smart tibial trial with an accelerometer will demonstrate abnormal soft-tissue balance and allow for either an incremental soft-tissue release or minor alignment changes via bone re-cuts. This technology may allow surgeons to provide their total knee replacement patients with better outcomes. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. The author received no grant support for the prepartation of this article. The author does receive royalties and consulting fees from OrthoSensor, the manufacturer of the smart trial. This paper is based on a study which was presented at the 30th Annual Winter 2013 Current Concepts in Joint Replacement® meeting held in Orlando, Florida, 11th – 14th December.
References 1. Vessely MB, Whaley AL, Harmsen WS, Schleck CD, Berry DJ. The Chitranjan Ranawat Award: Long-term survivorship and failure modes of 1000 cemented condylar total knee arthroplasties. Clin Orthop Relat Res 2006;452:28–34. 2. Noble PC, Conditt MA, Cook KF, Mathis KB. The John Insall Award: Patient expectations affect satisfaction with total knee arthroplasty. Clin Orthop Relat Res 2006;452:35–43. 3. Bullens PH, van Loon CJ, de Waal Malefijt MC, Laan RF, Veth RP. Patient satisfaction after total knee arthroplasty: a comparison between subjective and objective outcome assessments. J Arthroplasty 2001;16:740–747. 4. Dickstein R, Heffes Y, Shabrai El, Markowitz E. Total knee arthroplasty in the elderly: patients’ self-appraisal 6 and 12 months postoperatively. Gerontology 1998; 44:204–210. 5. Suda AJ, Seeger JB, Bitsch RG, Krueger M, Clarius M. Are patients’ expectations of hip and knee arthroplasty fulfilled? A prospective study of 130 patients. Orthopedics 2010;33:76–80. CCJR SUPPLEMENT TO THE BONE & JOINT JOURNAL
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6. Harvie P, Sloan K, Beaver RJ. Computer navigation vs. conventional total knee arthroplasty: five-year functional results of a prospective randomized trial. J Arthroplasty 2012;27:667–672. 7. Insall JN, Dorr LD, Scott RD, Scott WN. Rationale of The Knee Society Clinical Rating System. Clin Orthop Relat Res 1989;248:13–14. 8. Bellamy N. WOMAC Osteoarthritis Index User Guide. Version V. Brisbane, Australia 2002. 9. Walker PS, Meere PA, Bell CP. Effects of surgical variables in balancing of total knee replacements using an instrumented tibial trial. Knee 2013;21:156–161.
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10. Gustke K, Golladay G, Roche M, Elson L, Anderson C. Primary TKA patients with quantifiably balanced soft-tissue achieve significant clinical gains sooner than unbalanced patients. Adv Orthop http://www.hindawi.com/journals/aorth/aip/628695/ (date last accessed 30 July 2014). 11. Gustke KA, Golladay GJ, Roche MW, Elson LC, Anderson CR. A new method for defining balance. Promising short-term clinical outcomes of sensor-guided TKA. J Arthroplasty 2014;29:955–960.
Soft-tissue and alignment correction THE USE OF SMART TRIALS IN TOTAL KNEE REPLACEMENT
K. A. Gustke From Florida Orthopaedic Institute, Tampa, Florida, United States
Total knee replacement (TKR) smart tibial trials have load-bearing sensors which will show quantitative compartment pressure values and femoral-tibial tracking patterns. Without smart trials, surgeons rely on feel and visual estimation of imbalance to determine if the knee is optimally balanced. Corrective soft-tissue releases are performed with minimal feedback as to what and how much should be released. The smart tibial trials demonstrate graphically where and how much imbalance is present, so that incremental releases can be performed. The smart tibial trials now also incorporate accelerometers which demonstrate the axial alignment. This now allows the surgeon the option to perform a slight recut of the tibia or femur to provide soft-tissue balance without performing soft-tissue releases. Using a smart tibial trial to assist with soft-tissue releases or bone re-cuts, improved patient outcomes have been demonstrated at one year in a multicentre study of 135 patients (135 knees). Cite this article: Bone Joint J 2014;96-B(11 Suppl A):78–83.
K. A. Gustke, MD, Adjunct Clinical Professor of Orthopaedic Surgery University of South Florida, Tampa, Florida, and Florida Orthopaedic Institute, 13020 North Telecom Parkway, Temple Terrace, Florida, USA. Correspondence should be sent to Dr K. A. Gustke; e-mail: [email protected] ©2014 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.96B11. 34339 $2.00 Bone Joint J 2014;(11 Suppl A):78–83.
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Studies spanning 15 years have reported excellent 85% to 97% survivorship with total knee replacement (TKR).1 However, excellent survivorship does not equate to excellent patient reported outcomes and for example, Noble et al2 reported that 14% of their patients were dissatisfied with their outcome. The majority of these patients had problems with kneeling, squatting, gardening, lateral movements, and carrying loads. There is also a difference in the patients subjective assessment of outcome and the surgeons objective assessment.3 Dickstein et al4 reported that a third of TKR patients were dissatisfied, even though the surgeons felt that their results were excellent. Most of the patients who report lower outcome scores do so because their expectations are not being fulfilled by TKR.5 These low patient satisfactions are often puzzling. Modern TKR instrumentation can produce very accurate bone cuts and alignment. However, better alignment does not equate to better short-term function and outcomes.6 Radiological and physical examination of many of these less satisfied patients fail to reveal obvious correctable solutions. Our assumption is that many of these patients may be experiencing subtle soft-tissue imbalance and maltracking of the components that we have difficulty in assessing either intraor post-operatively. Surgeons are able to easily recognise knees that have gross instability and
both the patient and surgeon are in agreement as to why a poorer outcome has occurred. However, many more patients may have less detectable instability which the surgeon is not able to appreciate and the patient is usually unable to describe accurately. Current techniques for soft-tissue balancing rely on subjective feel and visual evaluation of the anteroposterior position of the femur on the tibia and medial and lateral gap assessment while varus and valgus stresses are applied. This technique is not very accurate. It is essentially an art with success related to the surgeons’ experience. In order to properly visualise the gaps, the surgeon needs to be able to see inside the joint. If the patella is everted or subluxated laterally, increased pressures are placed across the lateral compartment. When the knee is in flexion, without closure of the medial retinaculum, the posteromedial capsule is looser resulting in lower medial compartment pressures and a more posterior medial femoral contact point. The ‘smart tibial trial’ can eliminate the effect of inexperience on the judgment of amount of gap balance. It also allows the patella to be reduced and the medial retinaculum to be temporarily closed during assessment of component tracking and balance. We have used the OrthoSensor Knee Balancer smart tibial trial (OrthoSensor Inc., Dania Beach, Florida) to assess total knee balance in a series CCJR SUPPLEMENT TO THE BONE & JOINT JOURNAL
SOFT-TISSUE AND ALIGNMENT CORRECTION
79
Fig. 1
Fig. 2
Photograph showing the Verasense tibia liner with wireless microprocessors sensors to measure contact force and location.
Photograph showing the OrthoSensor Link station monitor with wireless connection to sensors
of patients (Fig. 1). The device has miniaturised integrated circuits and micro processers that are similar to those used in cell phones and it communicates to a computer via a wireless link (Fig. 2). The computer has a graphical interface which shows the pressure in pounds per square inch (psi) that are present and the location of maximal contact between the medial and lateral femoral condyles (Fig. 3). The pressure and component tracking can be assessed while taking the knee through a range of movement. These smart trials have the shape of standard implant trial devices, so it can be designed for use in different total knee systems. Currently, four major total knee systems make use of this product. They are single use, disposable, and fairly inexpensive. They are currently priced at US$ 459. They also allow the surgeon to use their preferred work flow, using classical gap balancing or measured resection for the bone cuts. The first generation of the OrthoSensor smart trial did not provide any alignment information. Standard instrumentation or computer navigation was relied on to obtain accurate component alignment. The OrthoSensor VERASENSE smart trial now incorporates accelerometers which show tibial axial alignment and overall hip-to-ankle mechanical axial alignment (Fig. 3) which is essential in making a decision on whether to perform a soft-tissue release. Knowing the alignment also gives the surgeon the option to slightly recut the distal femur or tibia avoiding having to perform a soft-tissue release.
The purpose of the multicentre study was to better define balance based on quantitative measurements and to determine if patients with quantifiably better balanced knee joints, achieved with the use of sensors, exhibited improved clinical outcomes. Patient pre- and post-operative anatomical alignment was obtained by measuring 18 inch standing radiographs of the knee. Clinical assessments and patient satisfaction was measured via American Knee Society Score,7 and Western Ontario and McMasters Universities Osteoarthritis Index (WOMAC) scores.8 A new Activity levels scoring system based on a 6 level 100 point scale was developed for the multicentre study in order to statistically quantify and better distinguish typical activities of a post-total knee replacement patient. The patients were asked to choose a category that best described their activity level from ‘bedridden’, ‘sedentary’, ‘semi-sedentary’, ‘light labour’, ‘moderate labour’, and ‘heavy labour’. They were provided with examples of what activities they would be expected to do in each category. Bedridden patients were either bedridden or confined to a wheelchair. A sedentary patient would have minimal ambulation or activity. Patients who can perform light house cleaning, white collar office work, or benchtype work would be considered semi-sedentary. A patient would be in the light labour category if they could perform heavy cleaning, assembly line work, or do light sports. ‘Moderate labour’ would consist of being able to lift up to 50 pounds (23 kg) and participate in moderate sports. Patients who could perform vigorous sports or lift 50 to 100 pounds (23 kg to 45 kg) were considered capable of ‘heavy labour’. Each descriptive category was designated with a numeric representation, at 20-point intervals: bedridden = 0, sedentary = 20, semi-sedentary = 40, light labour = 60, moderate labour = 80, and heavy labour = 100.
Our clinical experience with the balancing device A three-year prospective multicentre IRB monitored study was initiated in February 2012 for TKRs performed with the smart tibial trial using the Triathlon total knee system (Stryker, Mahwah, New Jersey). There were 135 knees in 135 patients with one year follow-up data. There were eight participating centres. VOL. 96-B, No. 11, NOVEMBER 2014
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Fig. 3b
Fig. 3a
Fig. 3c Graphic interfaces demonstrating a) compartment pressures and alignment, b) tight medial soft tissues causing elevated medial compartment pressures. Compartment pressures are more equalised after a medial soft-tissue release, and c) the tracking pattern of maximum contact pressure points as the knee is taken through a range of movement.
Use of the smart tibial trial was relatively new to all surgeons in this series and only one of the eight surgeons had previously used the Triathlon total knee system (Stryker) extensively before beginning the multicentre study. All surgeons were asked to use the device to assist in balancing the knees to their satisfaction. All knees were considered adequately clinically balanced by the surgeons at the end of the case based on their experience in assessing how the knees felt under varus/valgus and antero/posterior stress. When six month data was analysed on the initial study patients, it became evident that knees that had more equally balanced compartment pressures as determined by the smart trial were demonstrating better early outcomes. A sub-study of the initial multicentre study was initiated to determine if knees that had compartments more quantitatively balanced had better outcomes. Less than 15 PSI (103421 pascal) differences between the medial and lateral compartment pressures were arbitrarily chosen as the definition of satisfactory ‘balance’. A subsequent cadaver study by Walker, Meere and Bell9 validated that a 15 PSI (103421 pascal) difference correlates to optimal clinical balance. Using this definition, 18 (13%) of the 135 knees were ‘unbalanced.’ There was no difference in the ‘balanced’ or ‘unbalanced’ knees in age, sex, BMI, pre-operative alignment or range of movement. There
was enough statistical power for a comparison of the two groups (power 0.81). Most of the ‘unbalanced’ TKRs were performed in the early part of this series, suggesting the effect of a learning curve associated with new technology. The six-month results of the multicentre study have been reported and showed that satisfactorily ‘balanced’ knees according to our criteria had better patient reported outcomes and better activity scores.
The surgical technique when using the balancing device The femoral and tibial bone cuts are made as per the surgeon’s usual technique. The tibial components are inserted with the thickest standard tibial insert to eliminate the gap in the tighter medial or lateral compartment. With the knee in full extension, the VERASENSE smart trial is placed on the tibial crest at the level that is closest to being parallel to the tibial coronal plane to establish the tibial reference. The VERASENSE device is then placed into the knee using shims if necessary to increase the insert thickness. With the knee in full extension, the approximate anteroposterior slope of the tibia is graphically shown. With the knee near full extension and in perceived neutral rotation from the surgeons perspective, the tibial tray is rotated and CCJR SUPPLEMENT TO THE BONE & JOINT JOURNAL
SOFT-TISSUE AND ALIGNMENT CORRECTION
Fig. 4a
81
Fig. 4b
Graphic interfaces demonstrating a) high lateral compartment pressures near full extension (note 2.5° varus mechanical alignment) and b) compartment pressures more equalised after 3° valgus recut of the distal femur.
fixed so that the medial and lateral femoral tibial contact points have similar distances to the anterior tibial cortex. The contact point rotation value is registered. With the heel planted, the knee is flexed until the flexion dial on the screen indicates that the knee is in an optimal degree of flexion for the accelerometer. The hip is abducted and adducted until the accelerometer determines the varus/ valgus tilt of the tibial component. The leg is then placed in extension and elevated until the mechanical axis indicates a good position. While applying axial pressure on the foot, the hip is rotated until the hip/knee/ankle mechanical axis is determined. If the tibial alignment or mechanical axis is not within the surgeon’s desired acceptable range, the tibia or distal femur is recut. With the medial retinaculum held closed with two or three towel clips, the knee is taken through a range of movement while viewing the compartment pressures. In order to avoid any abnormal varus or valgus force, the distal thigh is supported with one hand and the heel is supported with the other. As experience was gained by the multicentre surgeons using the smart trial, the Triathlon Knee patients appear to have best visual stability when compartment pressures are between 20 and 30 PSI (137895 to 206843 pascal). If both compartment pressures are much lower than this, the knees appeared to be too loose. Having low compartment pressures will also increase the chance that inadvertently applied varus or valgus, while taking the knee through range of movement, will affect the compartment pressures. A thicker shim is then placed under the sensor trial. If compartment pressures are within 15 PSI (103421 pascal) of each other, the knee is deemed adequately balanced. Range of movement can be also performed with the tracking function on to confirm proper component kinematics. If compartment pressures are more than 15 PSI (103421 pascal) different a decision is made as to whether a bone VOL. 96-B, No. 11, NOVEMBER 2014
recut or soft-tissue release should be performed. If a bone recut is chosen, a recut of tibia is done if the tight compartment is tight in flexion and extension. If the compartment is tight only in extension, a distal femur recut is performed. If the compartment is tight only in flexion, a change of femoral rotation is performed. If the posterior cruciate ligament is too tight causing excessive medial and lateral roll-back in flexion, the tibia can be recut with more slope. A minimal recut of 2 mm or 2° can usually change a knee from an ‘unbalanced’ to a ‘balanced’ knee.8 If a soft-tissue release is preferred, the sensor loads will change sequentially as more releasing is performed. This facilitates the use of a pie crusting release technique with an 18 gauge spinal needle or a #11 knife blade. A knee tighter in extension than flexion would warrant the release to be in the more posterior aspect of the soft-tissue sleeve. If an outside-in release is performed, the retinaculum can be held closed with towel clips while the releases are performed while visualising load decreases on the monitor. After several piercings of the soft tissue, the knee is cycled through the range of movement to allow the tissues to stretch. If an inside-out release is performed, it needs to be gradual. The retinaculum is intermittently closed, and any change in forces is noted while the knee is ranged. Figure 3a is from a case with tight medial soft tissues. Figure 3b graphically shows knee balance after a piecrusting release was performed on the tight medial soft tissues. Figure 3c demonstrates the preferred component maximum contact point tracking pattern. Figure 4a shows a case demonstrating a tight lateral compartment mainly near full extension. The overall alignment of this knee is 2.5° of varus. Rather than performing a release of the iliotibial band, a 3° valgus recut of the distal femur was performed. Figure 4b shows balanced compartments and an accepted 1.7° overall valgus alignment.
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Clinical update One year data results from 135 knees in 135 patients in the multicentre study series has been published.10 The quantitatively ‘balanced’ knees continue to show statistical improvement over the six month data.11 The mean total American Knee Society Score of the ‘balanced’ group is 23.3 points higher than the ‘unbalanced’ group (p < 0.001); 179 + 17.2 and 156 + 23.4 for the ‘balanced’ and ‘unbalanced’ patients. The mean Knee Society pain score was 96.4 for the ‘balanced’ group and 87.8 for the ‘unbalanced’ group (p < 0.001). The mean Knee Society function score was 82.4 for the ‘balanced group’ and 68.3 for the ‘unbalanced’ group (p = 0.022). Mean pre-operative anatomical knee alignment was 4.9° of valgus for the ‘balanced’ group and 5.1° of valgus for the ‘unbalanced’ group. Mean postoperative anatomical knee alignment was 4.52° of valgus for the ‘balanced’ group and 4.22° of valgus for the ‘unbalanced’ group. The ‘balanced’ group had an 8 point higher improvement in WOMAC scores than the ‘unbalanced’ group (10 + 11.8 and 18 + 17 for ‘balanced’ and ‘unbalanced’ patients, p = 0.085). The ‘balanced’ group mean activity score was 68.6, which corresponds with light to moderate labour categories. The ‘unbalanced’ group mean activity score was 46.7, which corresponds to a lower semisedentary category. The difference between the mean activity scores was statistically significant (p = 0.015). The data that was most compelling, was that the Knee Society pain and functional scores, WOMAC scores, and activity levels for the ‘unbalanced’ knees at one year had still not reached what the ‘balanced’ knees had achieved at six months (combined Knee Society pain and functional score of 172.4, WOMAC score of 14.5, and activity score of 40.5). Discussion Despite improvements in total knee implant design and surgical technique, a significant percentage of TKR patients report inferior outcomes.1-5 With the assumption that softtissue imbalance may be responsible for some of the inferior outcomes and that surgeons have difficulty with accurately balancing TKRs with conventional surgical techniques, smart tibial trials have been designed. Use of smart tibial trials has provided the ability to quantitatively measure the anteroposterior and mediolateral balance of a TKR intraoperatively with the medial retinaculum closed. The sensors demonstrate imbalances through overly tight soft tissues. They will also demonstrate a malrotated tibial or femoral component. Soft-tissue releases can be performed sequentially while visualising their effect on balancing the compartment pressures. The latest generation sensors have accelerometers to visualise the tibial component and overall axial alignment. If an imbalance is present and a minimal bone recut will still keep alignment within acceptable ranges, soft-tissue releases can be avoided. The results reported from use of a smart tibial trial, with the ability to measure knee compartment pressures and alignment, are unique to this device.
The limitations of both the six-month and one-year reported studies are that the number of ‘unbalanced’ knees is relatively small.10,11 The majority of knees that were in the ‘unbalanced’ group were performed early in the study, during the learning curve for the surgeons using a new device and total knee system. As experience and data results became known to the surgeon group, fewer knees were left with a greater than 15 PSI (103421 pascal) difference between the two compartments. Another limitation is that the surgeons were not blinded as to which patients had ‘unbalanced’ or ‘balanced’ knees. Even though the surgeons knew at the time of surgery what ultimate differences in compartment pressures were present, they were still satisfied that the knees were clinically balanced and the knowledge of which group the patient was in was not readily available at the time of follow-up assessments. The outcome data for the most part was independent of surgeon bias and the patients did not know if they were in the ‘unbalanced’ or ‘balanced group’. Another limitation of the study is that there is no control group of total knee replacements without using the sensor performed by the same group of surgeons. Improved outcomes scores could be enhanced by the fact that patients knew they were having a surgery with a technologically advanced device. However, the patients did not know if they were in the ‘balanced’ or ‘unbalanced’ groups. Thus the placebo effect would be expected to be present in both groups. One-year reported results show that knees that are better balanced using a smart tibial trial have better pain, functional, and activity scores.10 A smart tibial trial with an accelerometer will demonstrate abnormal soft-tissue balance and allow for either an incremental soft-tissue release or minor alignment changes via bone re-cuts. This technology may allow surgeons to provide their total knee replacement patients with better outcomes. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. The author received no grant support for the prepartation of this article. The author does receive royalties and consulting fees from OrthoSensor, the manufacturer of the smart trial. This paper is based on a study which was presented at the 30th Annual Winter 2013 Current Concepts in Joint Replacement® meeting held in Orlando, Florida, 11th – 14th December.
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SOFT-TISSUE AND ALIGNMENT CORRECTION
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