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ABSTRACT
Majority of the cases of revision knee arthroplasties are due to improper soft tissue balancing. Good soft tissue balancing is as important as perfect bone cuts. There are several instruments which are used to achieve a perfect bone cut. But there is no instrument to obtain a good soft tissue balancing. Inadequate soft tissue balancing will lead to instability, pain and eventual failure. The technique of soft tissue balancing has to be mastered carefully. The soft tissue balancing should be assessed several times so that a rectangular flexion and extension gap is
achieved
J.Orthopaedics 2004;1(1)e2
Majority of the cases of revision knee arthroplasties are due to improper soft tissue balancing. Good soft tissue balancing is as important as perfect bone cuts. There are several instruments which are used to achieve a perfect bone cut. But there is no instrument to obtain a good soft tissue balancing. Inadequate soft tissue balancing will lead to instability, pain and eventual failure. The technique of soft tissue balancing has to be mastered carefully. The soft tissue balancing should be assessed several times so that a rectangular flexion and extension gap is achieved. The soft tissue balancing should be assessed preoperatively under anaesthesia with trials and originals.
Common clinical situation where a soft tissue balancing is needed are
1. varus deformity
2. valgus deformity
3. flexion contracture
4. defective patellar tracking
VARUS DEFORMITY:
Correction of contracture requires release of medial tight structures as a part of long standing deformity. There are two main reasons for a varus deformity. (1) The medial Tibial bone defect with lax medial structures. (2) Medial tightness without bone defect. The aim of medial release is to lengthen the medial soft tissue structures and to make it equal to the lax lateral structures. When the medial structures are lax, a perfect bone cut with minimal soft tissue release will balance the knee. When the medial structures are very tight with lax lateral structures, extensive soft tissue is often required for a perfect balancing. The medial soft tissue structures are elevated from the proximal Tibia and not from the distal Femur. The structures are released in a progressive fashion from anterior to posterior and from proximal to distal direction. The medial stabilizing structures are formed of mainly 4 anatomical structures. Medial Collateral Ligament, Semimembranosus tendon, Pes Ansarinus and Posterior Capsule. The medial collateral ligament is continuous with the periosteum of the Tibia. One should never try to separate the medial collateral ligament from the periosteum, which will result in total damage of the medial structures.
Medial capsular elevation is done by an osteotome, starting from the upper tibial border in the sub periosteal plane, medial to the medial margin of the tibial tubercle from proximal to distal direction, as far inferiorly as 15 cm, as and when needed, in a stepwise manner. During the whole process the tibia should be kept externally rotated. While doing this release, insertion of the pes ansarinus and semimembranosus may automatically get elevated, if not, it should be cut with a sharp knife. At the posteromedial margin of the soft tissue structures, there are usually some large veins which should be ligated to avoid a haematoma formation. If a perfect balancing is not achieved with the above procedure, one can make transverse zig-zag cuts in the medial capsular sleeve and medial periosteum for a better release. The medial meniscus should be excised leaving behind 2 mm of its rim so that the medial soft tissue sleeve is not totally damaged.
The above mentioned procedure is for an extensive varus contracture. In mild varus contractures, step by step release of the above structures is done, till a perfect balancing is achieved. The medial release should ideally be done in 90 degrees of flexion and full extensions. After every cm of release, balancing should be checked before further release, so that unnecessary soft tissue damage is avoided. When the varus deformity is more than 30 degrees, extensive medial release is usually needed. Removal of large medial femoral or tibial osteophytes helps in better balancing. If the deformity is not corrected with the above mentioned procedures, PCL should be sacrificed along with ACL.
Correct soft tissue balancing is reached when the medial and lateral sides are such that the knee can be stressed into varus or valgus of not more than 5 degrees. A perfect thickness of the implant design maintains the tension of the medial capsular structures, even after extensive release and will not result in medial instability.
VALGUS CONTRACTURE:
The release of valgus contracture is very difficult, when compared to the varus contracture. The principle is the same as a varus deformity in that step wise release of the lateral structures is done till its length equals the lax medial soft tissue structures. Osteophytes are usually not the cause for lateral ligament tightness, unlike the medial tightness produced by osteophytes. The principle of release of lateral structures is that, the release should be done from the distal femur and not from the fibula or upper tibia.
Valgus deformity could be due to a lateral bone defect with a lax lateral structures, or lateral soft tissue contracture without bone defect. The second one usually needs extensive soft tissue release. The bone defect can be easily managed by a perfect bone cut with minimal soft tissue release. The structures which become contracted in valgus deformity are iliotibial band, posterior capsule, popliteus tendon, lateral collateral ligament, lateral head of gastrocnemius, the biceps tendon and the lateral intermuscular septum.
Stepwise release on the lateral side includes cutting ITB transversely from the tibia, stripping the lateral collateral ligament and popliteus tendon from the lateral femoral condyle, keeping the knife close to the femur. In extensive valgus deformity, sometimes isolation and transfer of the common Peroneal nerve is usually needed(1,2). The lateral meniscus is excised. PCL is sacrificed if the deformity is severe. Upto 15 to 20 degrees of valgus deformity can be corrected by limited release, which includes keeping the popliteus tendon, lateral collateral ligament and biceps tendon intact.
More than 20 degrees of valgus contracture requires extensive release(3). In severe deformities the sharp dissection of the lateral capsule from the tibial rim, release of the popliteus tendon, lateral collateral ligament from the femur, the lateral head of gastrocnemius and elevation of the lateral intermuscular septum from the femur is required. In extreme cases lengthening of biceps tendon is done. In valgus deformity it is important to note that, one should not do a medial release, since all the medial structures are already lax. When an extensive lateral release is done, one should use a proper size poly ethylene so that there is no varus or valgus laxity of more than 5 degrees. Usually a thicker poly ethylene is needed after a valgus release than after varus release.
In short it should be stressed that valgus contracture release is a difficult procedure and may be associated with posterior erosion of the femoral condyle, lateral subluxation of the patella, increased Q angle and laxity of the medial collateral ligament. It needs perfect pre operative planning, better understanding of the anatomy and a good prosthesis.
FLEXION CONTRACTURE:
It is usually due to a tight and short PCL and a contracted posterior capsule in posterolateral and posteromedial corners of the knee. The release of the flexion contracture can be usually corrected by sacrificing the PCL, or lengthening the PCL along with posterior capsular release and capsulotomy. In some cases release of popliteus and semimembranosus tendon is also needed.
PATELLAR MALTRACKING:
It is a common problem in valgus deformity. It needs extensive release of lateral patellar retinaculum and release of the part of the patellar tendon from the superolateral pole of the patella, till a perfect tracking on the prosthesis is achieved
Reference:
1.Keblish PA, Valgus deformity in total knee replacement: the lateral retinacular approach Orthop Trans 1985:9-28.
2. Buechel FF, A sequential three-step lateral release for correcting fixed valgus knee deformities during total knee arthroplasty. Clin Orthop 1990:260:17
3.Insall JN. Surgical techniques and instrumentation in total knee arthroplasty. In: Insall JN, ed. Surgery of the knee, 2nd ed. New York: Churchill Livingstone; 1993.
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