Summary
Inability to obtain full extension following knee arthro- plasty is due to a combination of many factors. Some fac- tors are not under the control of the surgeon, and are re- lated to patient morphology and disease. Others are related to prosthesis design. Many, however, are directly related to the surgical technique and are therefore con- trollable by the surgeon. By attention to detail, the sur- geon can have a direct effect on these factors and can maximize extension.
Introduction
The major goal of total knee arthroplasty is relief of pain.
Almost as important, however, is the restoration of func- tion,and that function depends primarily on an adequate arc of motion in the knee. Extension and flexion follow- ing a knee arthroplasty are dependent upon a multitude of factors related to surgical technique, the implant used, the physical therapy program, and the patient him- or herself. This chapter will discuss these factors and de- scribe methods that the authors have used to maximize motion in extension.A subsequent chapter will deal with the subject of obtaining full flexion.
Why Do We Need Full Extension?
During normal gait, the knee is at full extension at the time of heel strike and then gradually flexes during stance phase and swing phase [1]. A patient whose knee cannot come into full extension must contract his quadriceps to prevent the knee from buckling during early stance, and this increases the work of walking.Whereas most patients after knee replacement have sufficient quadriceps strength to compensate in this manner when they first begin walk- ing,with continued walking quadriceps fatigue can result in a limp and anterior thigh pain [2].When the knee does not come to full extension, the limb is functionally short.
This can cause a limp as well as pain in the back and in the ipsilateral hip and ankle. For all these reasons, there-
fore,the goal for the total knee surgeon is to obtain full ex- tension in the reconstructed knee.
How Do We Determine
if the Knee Is Fully Extended?
At the onset,the authors would like to distinguish between two terms. Extensor lag refers to an inability to actively extend the knee to the point where it can be passively extended (it is the difference between passive and active extension).A flexion contracture, on the other hand, is an inability to bring the leg to full extension passively. Al- though regaining muscle power is important after knee replacement, very few patients have an extensor lag following primary surgery. It is a flexion contracture that we are most concerned with during knee replacement.
To examine for full extension, the patient should be re- cumbent with both legs exposed and the heel on the table.
If the knee is fully extended, the examiner should not be able to pass any of his hand behind the knee in the popliteal space.The greater the flexion contracture,the more fingers the examiner should be able to pass under the knee.
Full passive extension of the knee can appear limited if there is hamstring spasm or tightness (for instance, in the patient with discogenic disease),especially if the knee is tested with the hip flexed. It is for this reason that as- sessing extension while the patient is sitting with his leg hanging off the side of the examining couch often results in a false increase in the appearance of a flexion contrac- ture. If an exact measurement of extension is required, a lateral cross-table radiograph can be taken with the an- kle supported on a small box.
The standard method of recording knee range of mo- tion assigns zero degrees to the fully extended knee. A knee that has a 5° flexion contracture and can, for exam- ple, flex to 125° of flexion should be listed as having a range of motion of 5°-125°. The use of minus numbers should be reserved for degrees of recurvatum at the knee.
During surgery,these tests are difficult to perform be- cause the leg is encased in sterile drapes. A test has been used by the senior author that eliminates this problem:
The leg is lifted from the ankle and the ankle joint itself is
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passively dorsiflexed.Axial pressure is then applied to the sole of the foot. If there is a flexion contracture, the knee will suddenly flex.If the knee is at full extension,however, there will be motion.
Although the presence of pain can lead to a false eval- uation of joint motion, this relates predominantly to flex- ion. In a study performed at the senior author’s institu- tion,patients who were to undergo knee arthroplasty had an evaluation made of knee motion prior to and after the administration of their epidural anesthetic. Although an average of 15° more motion was obtained in flexion once the patient’s pain sensation had been eliminated, there was no significant change in extension.
What Factors in the Arthritic Patient’s Knee Can Cause a Block to Full Extension?
Lack of full extension is commonly seen in patients with ad- vanced arthritis who are candidates for knee arthroplasty.
In the author’s database of over 1500 patients undergoing TKA,the average block to full extension in patients with os- teoarthritis was 5°.In patients with rheumatoid arthritis the mean flexion contracture was 10.5°, while in patients with post-traumatic arthritis it was 14°. It is fairly intuitive that this pre-operative contracture must be corrected at surgery if a postoperative contracture is to be avoided.
For patients with a flexion contracture less than 10°- 15°, the culprit is usually anterior or posterior osteo- phytes (⊡ Fig. 30-1). Anterior tibial osteophytes are nor- mally removed when the proximal tibia is resected; how- ever, posterior femoral osteophytes, which can tent the
posterior capsule, are not easily visible during the surgi- cal exposure [3].
Posterior femoral osteophytes can be most easily re- moved once the proximal tibia and posterior femur are resected. A laminar spreader is placed medially, and the knee, in 90° of flexion, is distracted. A curved osteotome and angled curettes will remove the posterior osteophytes from the medial femoral condyle (⊡ Fig. 30-2). The posi- tion of the laminar spread is then changed to the medial side and a similar procedure is performed to remove any lateral femoral condylar osteophytes. Finally, a check should be made for any remaining osteophytes behind the posterior cruciate ligament.
For patients with a flexion contracture >15°, further releases are normally necessary. The next step should be elevation of the posterior capsule from the femur. The knee should be flexed maximally and laminar spreaders again placed between the femur and tibia. The posterior capsule of the knee can then be elevated for 1-2 cm from the proximal femur using a periosteal elevator [3]. For flexion contractures greater than 45° this same approach can be used to elevate the tendinous origins of the gastro- cnemius muscle medial and laterally.
In 1991, the senior author reported his results using a technique of transverse sectioning of the posterior cap- sule [4], a technique that had initially been described by Insall [5]. The safety of this procedure was based on the assumption that, in flexion, the posterior neurovascular structures displaced posteriorly away from the posterior capsule. In actuality, the reverse is true, as described by Zaidi [6] in 1995. With knee flexion, the neurovascular bundle is displaced anteriorly and can lie tethered against the posterior capsule. For this reason, posterior capsule sectioning should not be routinely used, lest inadvertent popliteal artery and vein damage occur.
An apparently simple surgical solution to correct a block to full extension would be to remove extra bone from the distal femur, i.e., a segment of bone greater than the distal thickness of the femoral component that will be inserted. Whereas an extra resection of 3-4 mm can at
⊡ Fig. 30-1.Multiple osteophytes
⊡ Fig. 30-2. Removing posterior osteophytes
times be beneficial to help correct a block to full exten- sion, further resection than this should usually not be performed.Doing so raises the joint line and adversely af- fects knee kinematics. It can also result in an extensor lag and, in the extreme, damage to the collateral ligament in- sertions on the femur.
What Factors in the Implant Itself May Lead to a Block to Full Extension?
All currently available total knee components, if they are properly aligned and positioned in the knee, allow com- plete knee extension. However, the surgeon has to be knowledgeable of the configuration of the tibial compo- nent in choosing the degree of posterior tibial slope that is to be created.For example,if the anterior portion of the tibial component is “built up”,as is the case with some ul- tracongruent inserts, a posterior slope of the resection will lead to anterior impingement and a block to full extension. In these cases full extension is normally obtained by resecting the tibia at 90° to its anatomical axis in the sagittal plane,rather than the usual 3°-5° back- slope.
What Factors of Surgical Technique May Lead to a Block of Full Extension?
Problems with the surgical technique itself can result in a block of full extension even in a patient who had full ex- tension prior to the knee replacement. This occurs be- cause of stuffing of the extension space.
The borders of the extension space are the resected surface of the distal femur and proximal tibia. The thick- ness of the space is related to the amount of bone and car- tilage that has been removed and the elasticity of the sur- rounding capsular structures. It is this space which must be filled with implants of proper thickness if the knee is to be stable in extension.Likewise,overfilling of this space can lead to a potential block of full extension.
In most situations, the surgeon will remove bone equal in thickness to the distal thickness of the femoral component to be inserted. Doing this positions the pros- thetic joint line at its proper proximal-distal level and en- hances knee kinematics and patellar tracking [7]. Since the distal thickness of different implants vary (normally between 8 and 12 mm), the amount of resection will vary dependent upon the implant itself.
The thickness of proximal tibia that is removed varies.
Most implant systems include some type of stylus device that senses the highest point on the “normal” tibial plateau and then positions the tibial cutting block a certain distance (normally 8-10 mm) below this. This method is applicable to many situations in which there is a “normal” (or at least
a less abnormal) side remaining on the tibial surface,i.e.,the lateral side in a patient with a varus deformity.
If one uses this method, the stylus must be placed at the lowest point of the normal side of the joint to judge the proper resection plane. For a varus knee, the stylus is placed laterally. Anatomically, the lateral tibial plateau is convex from front to back , and the lowest point is poste- rior, not in the center of the plateau (⊡ Fig. 30-3). Placing the stylus in the center of the plateau will result in insuf- ficient bone being removed and will lead to a stuffed ex- tension space. For a valgus knee, reference is made from the medial tibial plateau. Anatomically, that plateau is concave anterior to posterior so that the stylus there can be placed in the center of the surface.
The method of using the stylus on the lowest point of the good side becomes ineffectual in the patient with in- flammatory arthritis, where both the medial and lateral sides are often affected to the same degree.
In order to avoid this problem, the senior author has adapted a method [8] which is a combination of the mea- sured resection and extension space filling methods. The distal femur is resected as described above. The knee is then extended and tensed medially and laterally with laminar spreaders.A spacer block,equal to the sum of the distal dimension of the femoral component and the thinnest tibial component,is then set at the level of the cut surface of the femur. The inferior surface of the block marks the level for the tibial resection.
A block to full extension can also occur if the compo- nents are malpositioned. Although small degrees of mal- position usually cause no statistical difference in the arc of motion [9], larger amounts can result in a flexion con- tracture. For example, the senior author has seen cases where flexion of the femoral component greater than 15°-20° from the anatomical axis of the femur in the sag- ittal plane rendered the knee unable to extend fully.
A situation often occurs where the knee fully extends with the trial implants in place and a thigh tourniquet in- 196 IV . Surgical Technique
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⊡ Fig. 30-3. Sensor, posterolaterally
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flated. However, after the tourniquet is released and the incision is closed, there appears to be a flexion contrac- ture. This pseudo flexion contracture is secondary to a hemarthrosis.As such, in most cases it abates as the post- operative hemarthrosis abates.
Will a flexion contracture that remains at the end of the operation prior to tourniquet release gradually stretch out with time? Although there have been reports of this oc- curring [10, 11] this has not been the authors’ experience.
The degree of extension that is present with the implants in place and with the tourniquet inflated is most often the maximum extension that the patient will finally obtain.
On occasion, one encounters a patient who has un- dergone a revision operation during which his surgeon has “changed the polyethylene”, inserting a thinner com- ponent in order to preclude a flexion contracture. One must seriously question whether full extension was ever obtained at the original surgery. The soft tissues of the knee are viscoelastic. If one presses hard enough with the patient under anesthesia it may appear that the knee is ex- tending; however, as soon as the pressure is removed the knee will “spring back” into a mild flexion contracture.
During testing of the knee, it should be allowed to come to full extension without pressure being placed on the patella lest a false evaluation be obtained.
Although full extension is the goal, there are situa- tions in which full extension is not possible without marked shortening of the femur. Such might be the ex- pected situation if knee arthroplasty were performed in patients with flexion contractures >60° [12]. Such situa- tions were occasionally encountered in the early years of TKA, when patients would present to the surgeon after having been chair bound and nonambulatory for many years. With the knowledge of joint replacement that now exists among the medical and lay communities, seeing a patient at that late stage has become uncommon.For such a patient, pre-operative traction can often decrease the contracture to below 45°. The senior author has elected in those uncommon situations either to perform a femoral shortening and to accept an extensor lag or to allow a residual 10°-15° flexion contracture to remain rather than risking stretch injuries of the neurovascular structures.
The use of Botox injections into the hamstring muscles has recently been suggested for patients with severe flex- ion contractures secondary to cerebral palsy. This treat- ment may offer some promise for the patient with a severe pre-operative contracture in whom full extension is not possible at surgery without excessive femoral shortening.
Postoperative Factors
Despite proper surgery, a block to full extension may oc- cur following surgery. If, for example, the patient uses pil- lows or other bolsters under the knee on a repeated basis
during the first few postoperative weeks, a flexion con- tracture can develop. A flexion contracture can also de- velop if the patient is allowed to sleep in a continuous pas- sive motion (CPM) machine. Observations of patients sleeping in a CPM unit will often reveal that the knee joint moves from the axis of flexion of the machine and never comes to full extension. This problem has become less prevalent now that CPM machines are not routinely used 23 h a day, a method that was recommended during the 1980s.
Problems in Adjacent Joints
There are patients with bilateral knee osteoarthritis and severe angular or flexion deformities who, because of concomitant medical problems,undergo the knee arthro- plasty during two separate hospital stays, rather than simultaneously. The knee that has been operated upon first becomes longer than the contralateral side. To com- pensate for the leg length discrepancy, the patient will walk with the operated knee slightly flexed,and over a pe- riod of several months this can lead to a flexion contrac- ture.The treatment is to place a lift on the shoe of the non- operated leg until the time of its surgery. A similar prob- lem can occur if the patient has a severe hip flexion contracture secondary to coxarthrosis. It most cases it is beneficial to treat the hip first to allow full extension and then, at a later time, perform the knee arthroplasty.
References
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75-87
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Laskin RS (ed) Total knee replacement. Springer-Verlag, London, pp 41- 53
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