CONCEPTS UNDERLINING BALANCING IN TKA

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Good balance is essential for the effective pain-free and durable function of a TKA. It is important to recognize that knee balance is not simply a function of soft-tissue balance. Balance of the knee joint following TKA is influenced by, 1. Component size, level and alignment and related bone surgery 2. Articular geometry 3. Soft-tissue envelope ERRORS OF KNEE BALANCE There are innumerable ways to commit errors of knee balancing.

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Nội dung Text: CONCEPTS UNDERLINING BALANCING IN TKA

CONCEPTS UNDERLINING BALANCING IN TKA Wui K. Chung FRACS Good balance is essential for the effective pain-free and durable function of a TKA. It is important to recognize that knee balance is not simply a function of soft-tissue balance. Balance of the knee joint following TKA is influenced by, 1. Component size, level and alignment and related bone surgery 2. Articular geometry 3. Soft-tissue envelope ERRORS OF KNEE BALANCE There are innumerable ways to commit errors of knee balancing. Major errors may result in loss of motion, pain, instability, loosening or early wear of the prosthesis. The main categories of errors are • Component malalignment • Errors of patellar resurfacing • Alteration of joint level • Errors of soft-tissue balancing • Errors of Flexion-Extension space balancing Less severe errors may result in more subtle problems such as recurrent effusion, post-activity aches, and less than satisfactory range of motion. Some common errors of knee balancing are less recognized eg. • Extension loss – occurs frequently. To avoid this error the surgeon should aim to obtain 3 degrees of extension with the components in place. If only zero degrees of extension is obtained during surgery, there is a strong likelihood that a flexion deformity will ensue. Post- operative pain resulting in quadriceps inhibition and hamstring spasm if unattended will also result in development of a fixed flexion contracture • Flexion laxity – is a common error that is not recognized adequately by many practitioners. Laxity in extension is readily appreciated as patients with the problem suffer obvious instability. Mild flexion laxity may not cause instability symptoms but causes recurrent activity-related effusion, post-activity pains and increased wear rate. Flexion laxity often results from over-zealous soft-tissue release. • Over-zealous soft-tissue release – in the effort to fully expose the knee joint during surgery, surgeons often perform excessive soft-tissue releases. Release of most soft-tissue structures of the knee joint lead to greater expansion of the flexion space than of the extension space. Flexion-extension space discrepancy results from such error. Radical soft-tissue releases also cause more post-operative bleeding and pain, which may lead to poor post-operative range of motion. METHODS OF TKA There are two well-established methods to perform TKA. 1. The Independent Bone Cut 2. Flexion (Tibial) First Methods. Both methods have identifiable advantages and disadvantages. The Independent Bone Cut Method is perhaps the more popular method. This is the method popularized by Freeman-Insall. The surgical sequence in this method being 1. Femoral bone cuts according to prescription 2. Tibial bone cut according to prescription 3. Flexion-Extension space balancing
Technique The tibia is osteotomized conventionally – perpendicular to the long frontal axis of the tibia and to remove a prescribed amount of tibia bone. The femoral component is set at a level and rotational alignment according to a predetermined criterion. Femoral rotation is generally set according to the Trans-Epicondylar axis or the Whiteside line according to the surgeon’s preference and the amount of bone removed from the distal and posterior femur is matched to the thickness of prosthetic replacement. Soft-tissue balancing is performed subsequent to bone surgery to obtain a rectangular and equal flexion-extension spaces. Disadvantages This method suffers from the following disadvantages, 1. The amount of femoral external rotation is set arbitrarily and may not reflect correctly the native femoral rotation. There is frequently more femoral rotation in patients with osteoarthritis compared with normal non-arthritic joints. 2. The epicondylar landmarks are not easily identified and it has been reported that the rate of surgical error is high even in specialist centers. Olcott & Scott CORR 1999 – range +11 degrees and - 4 degrees 3. Soft-tissue releases to achieve collateral balance tend to increase flexion space more readily than extension space, potentially creating problems with equalization of flexion-extension gaps. Over expansion of the flexion requires the surgeon to resect more distal femur to increase the extension space to match the flexion space. This leads to an elevation of the joint level. The Flexion (Tibial) First Method was popularized with the introduction of the LCS knee and continues to enjoy popular use. In this method the tibia is resected first and then a measured rectangular flexion space is created under distraction of the flexion space. The femoral bone cuts are set parallel to the cut tibial surface. Thus in this method the rotational alignment of the femoral component is not pre-determined and is dependent on the soft-tissue balance. After creation of a measured and rectangular flexion space, an equal extension space is created to match the flexion gap. Advantages and Disadvantages Soft-tissue release in this method tends to be more selective and limited compared with the former method. There are however some disadvantages with this method as well. 1. Femoral rotational alignment being responsive to soft-tissue balance is also subject to errors created by the soft-tissue work 2. Femoral condyle hypoplasia not uncommonly encountered in lateral compartment arthritis presents may present difficulty assessing the amount of soft-tissue release 3. Overzealous soft-tissue release during the flexion phase of the operation may demand greater amount of bone to be removed from the distal femur during the extension phase of the operation. Joint level alteration is not uncommon using this method. Extension First Method This presentation introduces a less well-known method of performing TKA by an Extension First approach. The author was stimulated to evolve this method by, 1. Knowledge that with few exceptions, the release of ligaments and tendons about the knee leads to greater expansion of flexion space than extension space 2. The relative difficulty of correcting flexion contracture compared with tightness of flexion space 3. The frequency of joint level alteration caused by use of conventional methods The essential steps involved in this method being, 1. Standard paramedial arthrotomy, meniscectomies, anterior subluxation of tibia, removal of marginal osteophytes. 2. Resection of ACL +/- PCL according to the type of implant selected.
3. Early correction of deformity - Limited, selective soft-tissue release to correct malalignment of the Mechanical Axis in extension – performed under manual traction with care being taken to avoid over-correction. 4. Osteotomy of proximal tibia in standard fashion removing prescribed amount of bone from the less worn tibia condyle. 5. Distal femoral osteotomy performed under tension. Measured extension space created, factoring in the Femoral Valgus Angle and relation of distal femoral cut to the Frontal Epicondylar Axis (Mechanical Axis). The extension space created is thus rectangular and of a dimension to accommodate the anticipated thickness of tibial and femoral implants. 6. The knee is flexed to 90 degrees and a tensioner or spacer applied. The rotational position of the femur is checked against known axes such as the Trans-epicondylar Axis and Whiteside line. Minor adjustment may be required with selective soft-tissue release to ensure adequate external rotation – the aim being to set the Epicondylar Axis (or the Whiteside Line) parallel to the cut tibial surface. A measured amount of posterior femoral bone is then removed to create a flexion space than is rectangular and equal to the extension space.
This method of performing a TKA has in our experience resulted in effective correction of flexion contractures without significant alteration of joint level or flexion laxity. The author has observed that the Frontal Trans-Epicondylar Axis bears a consistent relationship with the Mechanical Axis of the femur, being perpendicular to this axis in the vast majority of patients. Applying this observation, the author is able to select the correct femoral valgus angle for resection of the distal femur without use of an intra-medullary rod. FUNDAMENTALS OF KNEE BALANCING The aims in TKA are simply to • To correct deviation of the Mechanical Axis and to centre the axis to the centre of the knee joint • Create a knee joint plane that is perpendicular to the Mechanical Axis • Osteotomize the tibial perpendicular to its’ Anatomical Axis – the native tibia surface and knee joint plane (frontal) is tilted in slight (average 3 degrees) varus. By convention during TKA the tibia is cut perpendicular to the tibia axis – this alters the native knee joint plane. In order to maintain parallel relationship between the femoral component and the tibia component at 90 degrees of knee flexion, the femoral component needs to be externally rotated an extra 3 degrees. For this reason the posterior condylar axis, which parallels the native tibia surface cannot be used to set the femoral component – an extra 3 degrees of external rotation is required if this axis is used for reference during TKA. Studies have shown that the Trans-epicondylar and the Whiteside line corresponds quite well to the required amount of rotation for the femoral component • Reproduce the correct joint level – It is necessary to understand that there is the Tibial Joint Level and the Femoral Joint Level. When the knee is properly balanced the 2 joint levels coincide. There are numerous suggestions about how best to assess joint level during TKA. 1. Meniscal line – where identifiable the meniscal line is a ready and accurate indication of the joint level 2. Tibial Joint level – is about 10mm proximal to the tip of the fibular head 3. Femoral Joint level – approximately 25mm below the lateral epicondyle and 30mm below the medial epicondyle
BASIC BONE CUTS Although there is anything up to a dozen bone cuts during TKA, there are only 3 basic cuts. 1. Tibia cut – affects flexion and extension space equally 2. Distal femoral cut – affects extension space only 3. Posterior femoral cut – affects flexion space only Knowledge of this is essential for effective balancing of the knee as bone cuts may be varied to deal with flexion/extension space issues. REPLACING WHAT IS REMOVED – Importance of knowing the knee system in use For most standard TKA the aim is to removed from the less worn condyle an amount of bone equal to the thickness of the prosthetic component. This rule is modified in certain situations to correct deformities that cannot be fully corrected by soft-tissue release. For example additional amount of bone may be removed from the distal femur to correct a fixed flexion deformity. Each millimetre of additional distal femur resection will correct 4 degrees of fixed flexion. Recognizing this principle it is thus important that surgeons performing TKA be completely familiar with specifications of the knee system they are using. THE PFC-SIGMA specifications With the Sigma fixed bearing system the required extension space is 1mm greater than the required flexion space. This rule holds true for all sizes up to size 5. Thus the minimum extension space required is 17mm (9mm distal femur + 8mm tibial thickness) and the minimum flexion space is 16mm (8mm posterior femur + 8mm tibial thickness). The thinnest tibial implant for the PFC-RP is 10mm which follows that the smallest extension space required is 18mm and the flexion space 17mm. KNOWING THE COMPONENT SPECIFICATIONS PFC-Sigma PFC Tibial Component •Tray thickness = 2mm •8mm insert = 2 + 6mm Minimum Space PFC-Sigma Using 8mm insert • 9 + 8 = 17mm distally • 8 + 8 = 16mm posteriorly ABOUT COMPONENT ROTATION Incorrect femoral or tibial component rotation affect knee balance and result in patellar instability and increased wear rate of the polyethylene insert. FEMORAL ROTATION The aim is to set the femoral component parallel to the tibia component when the knee is at 90 degrees flexion (discussed in the section above). Provided the tibia osteotomy is correct and the collateral ligament balance satisfactory, then the Trans-epicondylar Axis (or the Whiteside Axis) should be parallel to the cut tibia surface. The correct identification of the TEA and the WA can present difficulties. The WA may be difficult to identify if 1. the femoral sulcus is shallow due to lateral condylar attrition or hypoplasia
2. osteophytes formation in the intercondylar notch. Care should thus be taken to remove the osteophytes in order that the roof the notch may be clearly identified Mistakes are often made with location of the TEA. The lateral epicondyle is relatively easy to identify as it is a distinct bony prominence. The point to be used for the TEA is the highest part of the bone prominence and this is best located by running a finger proximal-distally on the bone. The point medially is more difficult to identify and there is debate about the best bone point to use. Berger et al identified 2 points 1. The Anatomical Medial Epicondyle – represented by the bone prominence on which the superficial medial collateral ligament originates 2. The Surgical Medial Epicondyle – represented by the sulcus from which the deep medial collateral ligament takes origin. He recommends use of this landmark to set the femoral rotation The practical difficulty during surgery is to identify the most prominent point of the medial epicondyle as this epicondyle unlike the lateral epicondyle does not rise to a peak. Rather the medial epicondyle resembles an inverted cashew nut without a raised peak. During TKA the author seeks the sulcus of the medial epicondyle by following the deep medial collateral ligament to its origin and uses a point 1mm above the sulcus as the landmark for the TEA. DEALING WITH BONE DEFORMITY Severe varus or valgus deformities may be associated with significant condylar bone loss particularly on the tibia side of the joint. It is a common and undesirable practice to perform the tibia osteotomy at the lowest level of the bone defect. This leads t a number of potential problems. 1. Component size mismatch - the tibial implant size may end up smaller than the femoral component size. Although most manufacturers advise that the femoral and tibial component may be a size up or down, one should be aware that size mismatch can adversely affect the quality of articular contact. It is best to avoid a mismatch of sizes. 2. A low tibial resection may result in impingement of the tibial tray against the fibular head The preferred technique is thus to build-up rather than resect down.
•Too much bone removed •Lateral joint pain •Component mismatch
GUIDELINES TO SOFT-TISSUE BALANCING Surgeons not infrequently carry out excessive amount of soft-tissue release during surgical exposure and balancing of the knee. Such over-zealous release may result in • Over expansion of the flexion space which in turn demand further bone resection or soft- tissue surgery to increase the extension space in order to create a “balanced” situation • Joint instability • Post-operative bleed and pain • More scarring and loss of motion Care should be taken to be selective about the soft-tissue structures to release and to titrate the amount of release to the degree of correction required. USE OF TENSIONING DEVICE Although there is potential value in the use of a tensioning device, it is far more important that the surgeon assess joint space and tissue tension under similar conditions. It is a very common error, whether using spacer blocks or tensioners, for surgeons to perform the evaluation in flexion with the weight of the thigh on the knee whilst performing the evaluation in extension without the same weight on the knee. The weight of the thigh should be eliminated during the examination in flexion for the testing to be accurate. FLEXION-EXTENSION SPACE BALANCING Achieving rectangular and equal flexion / extension space is a fundamental requirement for successful TKA. Some helpful points to register in this regard are, 1. FIXED FLEXION - It is more difficult correcting a fixed flexion deformity than it is to correct tightness in flexion because release of most soft-tissue structures in and about the knee leads to greater expansion of the flexion space than the extension space. Mild flexion deformity can be corrected by posterior capsular release. Moderate and severe deformity requires additional amount of distal femoral resection. Very severe fixed flexion deformity of more than 50 degrees should be managed with caution because of the stretch risk of injury to the Lateral Popliteal Nerve. Most cases can be managed by • Radical posterior capsular release • Release of Gastrocnemius at insertions • Additional resection of distal femur • Knee splintage at 50% correction and serial adjustment as permitted by function of Lateral Popliteal Nerve over a period of 7-10 days to minimize risk of nerve palsy 2. LIMITATION OF FLEXION – is relatively easier to correct. It is essentially to first identify the anatomical factors causing the limitation of motion eg. Osteophytes, notch occlusion, capsular/ligamentous contracture, quadriceps contracture etc. In general such joints should be treated with a PS TKA as excision of the PCL substantially increases flexion space. A useful algorithm is • Comprehensive posterior and intercondylar osteophyte removal • Ensure 4-6 degrees of posterior slope to the tibia osteotomy • Avoid patellar over-stuffing • Downsize femoral component or use “Hi-Flex” design with reduce posterior condylar radius if that is available • Multi-puncture quadriceplasty for muscle contracture • Epicondylar osteotomy to release collateral contracture 3. VARUS DEFORMITY – The amount of surgery necessary to correct this deformity vary according to the stiffness of the deformity and can be judged by how well the deformity corrects with passive testing. Mild varus deformity of less than 5-6 degrees is generally corrected by the standard medial capsular release required for exposure of the knee joint and
subluxation of the tibia. Moderate deformity of up to 12 degrees will usually require release of the semi-membranosus tendon at its insertion on the posteromedial condyle of the tibia. Additionally some cases require partial release of the deep medial collateral ligament. Correction of the deformity should be assessed with the knee in extension as well as flexion. Persistent varus deformity in flexion is usually associated with tightness of the mid and anterior portion of the deep MCL and only this portion of the ligament require release. Varus deformity in extension is usually associated with tightness of the posterior portion of the ligament. The author prefers selective and careful titration of ligament release using the multi-puncture method. A tensioner is useful for this method of selective release and one should be cautious to increase distraction in graduated and careful steps so as to avoid over- release. Over release of collateral ligaments are a very difficult complication to treat. Severe varus deformity of more than 12 degrees may require release of the superficial MCL. The author prefers release of this ligament by osteotomy of the medial epicondyle to releasing the ligament at its distal attachment, as this method provides more control and avoids the problem of over-release. Severe varus deformity is often associated with bone loss of the posterior portion of the medial tibial condyle. The bone defect can be managed by autologous bone graft or metal augment (older patient). 4. VALGUS DEFORMITY – there is no agreement amongst practitioners about the best method to correct the valgus. The author prefers using the paralateral approach for moderate and severe deformity and finds the approach suitable for applicable for most patients with the exception of the obese. It is necessary in the preoperative examination to determine if the deformity is • Present in extension, flexion or in flexion as well extension • Correctable or not to passive stress – tight or loose deformity This information arms the surgeon with knowledge about the soft-tissue structures that are likely to require attention during the TKA. Deformity in extension is generally correctable by simple release of the Ilio-tibial band (the author preferring to perform the release at the insertion at the Gerdi tubercle). Deformity in flexion is usually associated with bone loss and the majority are “loose” deformity. The algorithm for treatment is • Mild (20) – Popliteus tendon, lateral epicondyle osteotomy Care should be taken before a decision is made t release the Popliteus tendon as the action will result in quite dramatic expansion of the flexion space. In the author’s experience the need for Popliteus tendon release is uncommon. Correction of major valgus deformity is associated with a significant incidence of Lateral Popliteal Nerve injury. 5. UNSTABLE JOINTS – Major collateral ligament instability presenting for TKA may be due to bone loss or ligament incompetence. The former is managed by management of the bone loss and little additional ligament surgery is required. Instability due to ligament incompetence may be treated by • Ligament advancement or augmentation (the author recommends the former) • Use of constrain prosthesis – TC3 or SROM rotating hinge depending on the degree of instability
“MINOR MISTAKES” THAT ARE UNFORGIVING It is not possible even amongst the most skilled surgeons to perform every operation to perfection. Mistakes are made, though some are more harmful than others. There are few “minor” technical errors that tend to cause significant symptoms. Some of these technical pitfalls are discussed below. 1. Component malrotation – Internal rotation of the femoral component is poorly tolerated and the risk of consequent patella dislocation is high. Internal rotation of the tibial component is better tolerated especially when using a mobile bearing device. 2. Component overhang – Lateral overhang particularly of the tibia component is poorly tolerated. In my experience patients with more than 2mm of lateral overhang of the tibial component tend to suffer from soft-tissue impingement symptoms. This error is made usually when the surgeon endeavours to provide full cover of the cut tibial surface by using a larger than necessary tibial tray. 80% of good host bone contact is all that is necessary to provide a stable platform for fixation of the prosthesis and there is no need to fully cover the cut surface. Although there is value in lateralizing the tibia and femoral component in order to benefit patella tracking, care must be taken to avoid lateralizing beyond the bone edge. 3. Lateral placement of patella component – the articular centre of the native patella is medial of the anatomical centre because the lateral facet normally accounts for 60% of the articular surface. Thus when positioning the patella component, the prosthesis should be centred about 5mm medial to the mid-point of the cut patella surface. The plane of the patella osteotomy is also an important consideration. Studies have shown that oblique osteotomy of the patella is associated with higher loosening rate of the prosthesis. To achieve a horizontal cut of the patella it is necessary to resect more of the medial facet than lateral facet. 4. Tibial Osteotomy error – the most common error of tibial osteotomy is the varus osteotomy. This is usually the result of displacement of the tibial cutting block by the patella tendon. It is well documented that error of more than 3 degrees is associated with higher incidence of tibial component loosening. When such error is committed in the obese patient the margin of error is even smaller. 5. Patella tendon avulsion – this is a dreadful complication that can occur early or late. It is caused by traction on the patella tendon insertion when exposing a tight knee. The complication may be avoided by adopting one of these techniques, • Avoid eversion of the patella till femoral and tibia bone resections are completed • Adequate resection of the fat pad – which contributes substantially to reducing tension of the patella tendon. This is particularly so when performing revision surgery. • Quadriceps snip • Protective pin inserted through patella tendon insertion – this popular method is unlikely to be helpful