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LEGACY POSTERIOR STABILIZED/CONSTRAINED CONDYLAR KNEE FEMORAL COMPONENTS/ART - Class 2 Recall

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and moments that in a constrained prosthesis would be ..

PCL-retaining revision arthroplasty could theoretically be used to revise early failure of a unicompartimental knee without major osteolytic lesions: integrity of collateral ligaments and a competent PCL are mandatory requirements. PCL resection revision arthroplasty can be performed releasing the posterior cruciate ligament and substituting it by an ultracongruent polyethylene insert. In the PCL substituting revision arthroplasty, the function of the PCL will be substituted by a standard cam and post-mechanism: integrity of collateral ligaments and a competent posterior capsule are mandatory requirements. Semi-constrained revision arthroplasty requires the presence of some collateral ligaments function and posterior capsule competency. The higher central post eventually increases stability in the presence of unbalanced gaps, but its function is related to the contemporary presence and continuity of the collateral ligaments. Hinged revision arthroplasties are designed to substitute collateral and cruciate ligaments, bone loss, and incompetent posterior capsule. Rotating hinge designs have the theoretical advantage of reducing bone implant stresses and early aseptic loosening.

(revision to a PS prosthesis or to a semi-constrained implant).

Fixation in the knee, as in the hip, is accomplished with both cemented and uncemented techniques. There is significant controversy surrounding which type of device provides optimum fixation. In brief, many surgeons favor noncemented knees in younger active patients where the biological remodeling potential exists at the bone prosthetic interface to re-anneal any microfracture incurred during vigorous activity. Micro-fractures at the bone prosthetic or bone cement interface can result in loosening, but press fit knees can theoretically heal these small breaks with biologic “spot welds”. This theoretically provides for better long term implant survival in such patients. However, other surgeons prefer cemented fixation especially in elderly osteoporotic, rheumatoid patients, or patients subjected to prior irradiation who have little biologic potential for ingrowth. Such patients might precociously loosen with early failure because the never developed sufficient biologic fixation in the first place. This reasoning is supported by the well established fact that uncemented press fit acetabular components fail with aseptic loosening at a significantly higher rate than cemented components in patient treated with irradiation to the pelvis. These surgeons also argue that cemented implants provide immediate fixation allowing for earlier safer weight bearing and are less expensive than the newer press fit knees. Interestingly, one randomized controlled trial of cemented versus cementless press-fit condylar total knee replacements in 501 replacements showed a ten-year survival of 95.3% and 95.6% in the cemented and cementless groups, respectively. (39) Another study reported a higher revision rate in press fit knees but similar function and outcomes scores. (40) Nevertheless, these similar results will only assure that the debate will continue for the time being in the knee.

Dislocation of a constrained knee prosthesis.2 case ..

24/04/2012 · [REVISION TKR] Zimmer Rotating Hinge Knee Prosthesis

While examining the patient it is important to note any previous incisions, limb length discrepancy, hip and foot deformity. Inability to flex the ipsilateral hip, such as with a hip fusion, can make operative positioning difficult during TKA. Rheumatoid arthritis patients often suffer from concomitant arthritis of both the hip and knee. Generally, hip arthroplasty should be performed prior to knee arthroplasty, because hip flexion is needed while performing a TKA, and the hip is more tolerant of delayed rehabilitation than the knee is. A valgus foot puts a valgus strain upon the knee. Considering correction of ankle deformity prior to TKR is advisable. If operative correction of the ankle is not done, then standard tibio-femoral cuts could leave the ankle with a severe valgus deformity. In rheumatoid patients, a deformity of the foot may be the main cause of the deformity of knee. A patients vascular status should be carefully evaluated prior to proceeding. If a patient does not have palpable pulses distally with good doppler signals, then the surgeon should consider not using a tourniquet during the case to avoid vascular embarrassment. Additionally, if the patient has diabetes with pulses that are not palpable but are dopplerable or the patient is a non-diabetic without doppler signals, then a vascular surgery consult should be obtained to explore options for augmenting perfusion. (46)
Complete preoperative radiographs to evaluate the knee include: 1)Standing full length AP film from the hip to ankle, this film allows for evaluation of mechanical axis of the entire lower extremity as well as evaluating for prior trauma or neoplastic lesions in the adjacent areas. 2) Standing extension lateral on large cassette (14” X 17”) 3) Flexion AP and lateral on large cassettes to evaluate the posterior aspect of tibio-femoral joint as well as sagital plane deformity. Flexion films will often reveal a more extensive pattern of DJD and joint space narrowing than is shown on the extension films of the knee. 4) Merchant’s or sunrise views to evaluate the patellar-femoral joint. Preoperative radiographs reveal the patients current mechanical axis and alert the surgeon as to the possibility of needing to perform soft tissue releases to balance the knee after bony cuts have been made in an anatomic fashion. Normal mechanics of the lower extremity have the tibia in slight valgus (3 degrees) relative to the femur with the tibial mechanical and anatomic axis being coincident. Figure 36. The aims of TKA include restoring the normal mechanical axis of the leg while balancing the soft tissue of the knee to optimize its biomechanics. Plain films are invaluable in achieving these goals and are required for completing device specific templating of the knee to in accomplishing these goals.

Total elbow replacement for distal humerus fractures and traumatic deformity: results and complications of semiconstrained implants and design rationale for the Discovery Elbow System.

Dislocation of a posterior stabilized total knee prosthesis.

Another reason for extension instability following primary TKA is an excessive intraoperative medial or lateral soft tissue release leading to varus‒valgus instability in extension: this scenario can be treated increasing the level of implant constraint (revision to a PS prosthesis or to a semi-constrained implant). The level of constraint in this setting is usually chosen on the degree of soft tissue laxity: Azzam et al. [] do not recommend anymore any soft tissue reconstruction, like a collateral ligament repair or soft tissue advancement, because of a high failure rate.

Figure 3 Figure 38. Graph demonstrates average polyethylene wear rates (mg/million cycles) when comparing sterilization techniques using gamma irradiation in air vs ethylene oxide vs gamma irradiation in an inert vacuum environment.
The thickness of the polyethylene is important to prevent catastrophic failure. Polyethylene thickness must be at least 8 mm to keep the yield strength of the ultrahigh molecular weight polyethylene above the contact stresses experienced in the typical TKA. In larger patients, or patients that are young and more physically active larger polyethylene implants should be considered and accounted for when balancing the knee. Obviously the thinner the poly insert the smaller its yield strength and the critical cutoff is about 8-10mm of thickness.

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Long-term survival of semi-constrained total knee ..

This review article analyzes recent reports comparing the functional outcomes of posterior stabilized, unlinked constrained, or fully constrained prosthetic design options in the management of bone loss and instability in revision TKA.

prosthesis, knee, patellofemorotibial, semi-constrained, ..

Surgical Technique
Several exposures have been adapted to TKR. The most commonly used approach in TKA is the medial parapatellar approach. Figure 37. This exposure is started with a longitudinal midline or medial parapatellar skin incision. It is important to keep in mind that the superficial blood supply comes mainly from the medial side of the knee and this approach will leave the patient with some numb spots on the lateral aspect of the knee. Additionally, in revision surgery where a much larger exposure is required, a more lateral incision should be used to not “outflank” the blood supply to the skin flaps above. During deep dissection an arthrotomy is made on the medial side if the patella leaving approximately 2-3mm of retinacular tissue to close the joint with. This arthrotomy is carried proximally and distally in the tendonous tissue until the patella can be everted laterally. It is important to avoid dissecting into the vastus medialis as well as leaving enough tendounous tissue to sew on both sides of the arthrotomy. Scissors can be used to transect the patellafemoral ligament off the femur and a synovectomy is performed at this point (if appropriate). Some surgeons trim the anterior fat pad but do not remove it entirely to provide extra vasculature and padding to the surrounding soft tissues. Patellar resurfacing can be performed early in the exposure in order to thin the patella and facilitate the eversion. After the patella is everted and the knee flexed the tibial insertion of the ACL can be incised which allows the tibial to be subluxated forward to allow complete visualization of the plateau surfaces. A rongeur is used to remove marginal osteophytes from the femoral condyles, patella and tibia as well and intercondylar space. Osteophytes from intercondylar notch of femur should be removed particularly if prosthesis is designed to retain PCL as the may abrade and impinge on the PCL. A lateral parapatellar approach is sometimes used in very valgus knees where there will be significant resection off of the medial femoral condyle. It is also occasionally employed for difficult primary /revision knee replacement, to protect the tibial tubercle from avulsing on lateral rotation of the patella.

Does a non-stemmed constrained condylar prosthesis predispose to ..

Well-recognized causes of early instability after TKA include mismatch between the flexion/extension gaps, improper coronal and rotational component alignment, and intraoperative loss of ligamentous integrity. On the other hand, late instability is usually linked to loss of fixation with concomitant moderate-to-severe bone loss and polyethylene wear. Since the ultimate goal of revision arthroplasty is anatomical and functional restoration of the knee joint, the concomitant presence of bone loss and soft tissue laxity might reduce the chances of final optimal outcome. During revision TKA, increasing levels of prosthetic constraint may be required to maintain stability of the knee joint.

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