Low Contact Stress (LCS) Complete Knee System in Revision Surgery

By Craig N. Lippe, MD; Lawrence S. Crossett, MD

Abstract

Revision total knee arthroplasty (TKA) should offer the same benefits to patients as primary TKA. As in primary TKA, a main objective of revision TKA is to reduce pain and restore functional range of motion. There are several potential causes of total knee failure, but the principles of repairing each of them is similar. The long-term success of the low contact stress knee system in primary TKA is well established, and clinical evidence for revision TKA with the low contact stress knee is promising.

Most patients who undergo primary total knee arthroplasty (TKA) achieve significant pain relief and restoration of function.¹ However, a small percentage of patients will have continued pain, instability, or stiffness for a variety of reasons. The causes of total knee failures include component loosening, infection, osteolysis, instability, patellofemoral problems, polyethylene wear, lack of motion, and malposition of the components.² Revision knee surgery presents unique problems to the surgeon including bone loss and soft tissue deficiencies that must be addressed at surgery.

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There is the potential for significant patient morbidity, including extensor mechanism disruption,³ wound healing problems,⁴ and an increased risk of infection.⁵ Revision knee surgery also presents a considerable health care burden in terms of cost and use of resources, especially in large referral centers.⁶ Ideally, any revision should be unnecessary. However, once presented with a failed implant, the surgeon must have a revision system available that has the options and flexibility to handle issues of bony loss and soft-tissue deficiencies and one that provides the patient with a stable, well-fixed functional knee.

Total Knee Revision Concepts

TKA revision surgery requires a thorough understanding of normal and pathologic anatomy. Implant failure is frequently accompanied by varying degrees of bone and soft-tissue loss. Additional difficulties include scarring, flexion-extension gap imbalance and changes in the anatomic joint line. The basic principle of revision arthroplasty involves creating a kinematically stable knee that is well fixed and well aligned. This involves the management of the residual bone and soft-tissue loss. Any revision knee system must adequately address these defects with varying levels of prosthetic constraint, component augmentation, diaphyseal stems, and metaphyseal sleeves. An implant that combines this interoperative flexibility with the benefits of a rotating platform should optimize patient outcomes. If the knee surgeon accepts the concept of low contact stress arthroplasty, it is only logical that these principles should be applied in clinically compromised patients.

Benefits of the Rotating Platform

The clinical history of the low contact stress knee is well documented,^7,8 and the use of a rotating platform in revision knee surgery draws on the advantages of rotation (Figure 1). Unidirectional motion reduces polyethylene wear and subsequent lysis. Rotation reduces the loosening forces at the bone/prosthetic interface, allowing for stable long-term fixation. The congruency of the bearing gives the patient excellent stability in gait. These features, in combination with a revision system that allows the surgeon to address the deficiencies encountered in the knee surgery revision setting, should provide an optimal knee revision system.

Figure 1: The LCS Complete Revision Knee System allows rotation between the polyethylene insert and tibial tray.	Figure 2: Excessive polyethylene post wear.

McEwen et al⁹, Mars et al,¹⁰ and Wang et al¹¹ have documented the benefit of unidirectional motion versus multidirectional motion in the reduction of volumetric polyethylene wear. The unique design of the rotating-platform mobile-bearing knee translates complex input motions into more unidirectional motion, resulting in a reduced wear rate due to decreased shear in the polyethylene. In contrast, the rotation of a fixed-bearing knee occurs entirely at the femoral-insert articulation, resulting in multidirectional motion and increased shear forces, which produces greater polyethylene wear. The unidirectional motion encountered with the rotating-platform mobile-bearing knee reduces the potential for long-term osteolysis by minimizing volumetric polyethylene wear.

Figure 3: The LCS Complete Revision Knee System.

Another problem that rotation potentially addresses is that of polyethylene post wear encountered with posterior-stabilized or varus- or valgus-constrained, fixed-bearing knees. Femoral rotation is resisted by the post in a fixed-bearing knee, resulting in high stresses and wear (Figure 2). The rotating-platform prosthesis allows the post to rotate with the femoral component.

Stable fixation of prosthetic components is extremely important to the long-term success of both primary and revision knee arthroplasty. Many factors contribute to the ultimate result. The surgical technique and the quality of bone are arguably the most important factors. The biomechanical characteristics of the prosthesis and the activity of the patients are the next most important factors in determining outcome. The geometry of any prosthetic design can and will restrict movement. Any force caused by motion in the knee that is restricted by the prosthesis is transmitted to the bone/prosthesis and tibial polyethylene/tibial tray interfaces. The greater the intrinsic constraint, the greater the force conducted to these interfaces. Thus, there is a greater potential for loosening of the prosthesis and backside wear.¹² A rotating-platform prosthetic design uncouples these forces, minimizing the rotational and shear forces that are conducted to the bone/prosthetic interface, and allows for the compressive loading of bone.¹³

LCS Complete Knee System in Revision

The LCS Complete Knee System for revision (DePuy Orthopaedics, Warsaw, Ind) was designed to bring the kinematic advantages of the LCS Complete Primary system to revision surgery (Figure 3). The congruency of the bearing gives excellent stability in gait. The mobile bearing allows rotation to occur at the polyethylene bearing/tibial component interface, lessening the stress at the prosthetic interface, which is often compromised in the revision setting. In addition, the unidirectional wear pattern of the rotating platform significantly reduces volumetric polyethylene wear and subsequent lysis or polyethylene failure. Superior patellar kinematics aid in postoperative patella tracking. The addition of diaphyseal stems, tibial and femoral metaphyseal sleeves, tibial and femoral augmentation, and varying degrees of constraint (Figure 4) offers the surgeon intraoperative flexibility when managing the diverse levels of bony and soft-tissue loses encountered in revision TKA.

The Mobile-bearing Tibial Revision Tray

A tibial revision system must allow the surgeon the options of adjunctive stem fixation, methods to manage bone loss, and various levels of prosthetic constraint. The mobile-bearing tibial revision tray (Figure 5) serves as a stable and versatile foundation in the revision knee setting by offering abundant intraoperative options and a platform to compensate for severe bone loss and soft-tissue deficiencies.

The mobile-bearing tibial revision knee tray encompasses a wide array of options to assist the revision knee surgeon when handling bony and soft-tissue deficiencies. There are multiple sizes of tibial components to allow for proximal tibial coverage. Stepped metaphyseal sleeves allow for the filling of bony defects and superior metaphyseal compressive loading. Trial sleeves are sequentially broached until bony defects are overcome and solid fixation in the metaphyseal bone is achieved. Tibial augmentations are also available to manage uncontained bony defects, allowing the surgeon to achieve a stable platform on good bone for excellent fixation. Cemented or uncemented tibial diaphyseal stems are available in various lengths and diameters, offering the knee surgeon flexibility in achieving a stable construct. The mobile-bearing tibial revision tray allows the revision surgeon to accomplish the goals of filling substantial bony defects, restoring the joint line, and providing a strong foundation for solid fixation with compressive loading of bone.

Figure 4: A variety of augments, metaphyseal sleeves, diaphyseal stems, and polyethylene inserts are available with the LCS Complete Revision Knee System.

An additional goal of revision surgery is to use only the constraint necessary when dealing with soft-tissue deficiencies. The mobile-bearing tibial revision knee tray accepts a wide range of bearings with increasing levels of constraint to cope with soft-tissue deficiencies. The tibial platform may be used with rotating-platform inserts from the LCS Complete (a simple rotating-platform insert), the LCS Complete Rotating Platform posterior-stabilized insert, or varus- or valgus-constrained inserts. All of these inserts share the same articulation and central stem geometry with the clinically successful LCS knee. In addition, the revision surgeon has the option of using inserts from the P.F.C. Sigma RP and Sigma TC3 sets in combination with the P.F.C. Sigma revision femoral component. Finally, the revision surgeon has the option of using a rotating platform-hinged insert from the Limb Preservation System, which is compatible with the S-ROM Noiles Rotating Hinge femoral component and the Limb Preservation System distal femoral component. The knee revision surgeon now has the intraoperative ability to manage a wide range of soft-tissue deficiencies using a single tibial platform.

LCS Complete Revision Femoral Component

A revision femoral prosthetic system must allow the restoration of anatomic alignment and functional stability, stable fixation of the revision implants and reestablishment of the joint line. In addition, it is of paramount importance, especially when using a mobile-bearing revision insert, for the revision surgeon to be able to adequately control the flexion gap. The femoral component in the LCS Complete Knee System for revision (Figure 6) accepts bearings of varying constraint and offers the surgeon the options of adjunctive stems, metaphyseal sleeve fixation, and distal and posterior augmentation.

Figure 5: The M.B.T. Revision Tray with available augmentation.	Figure 6: The LCS Complete Revision Femoral Component with available augmentation.

The femoral component in the LCS Complete Knee System for revision is available in multiple sizes and allows the surgeon to match the femur size to the tibia. Femoral augments are available to fill bony defects, re-establish the joint line, and balance the flexion-extension gaps. Distal femoral augments are available in 5-, 10-, and 15-mm sizes, and posterior femoral augments are available in 5- and 10-mm thicknesses. Femoral metaphyseal sleeves are available to fill contained bony defects and provide a stable base for solid fixation of the implant in good bone. Finally, both cemented and uncemented stems are available in various sizes to achieve stable fixation.

Surgical Technique

Preoperative Planning

Revision total knee arthroplasty begins with thorough clinical and radiography evaluations. Physical evaluation includes examining the soft tissues and noting previous incisions. Range of motion, motor strength, condition of all neurovascular structures, ligamentous stability and integrity of the extensor mechanism are evaluated.

Radiographic evaluation includes biplanar views of the knees and tangential views of the patella to assess the present implant and evaluate bone stock. Full-length radiographs are useful in assessing overall alignment. Templates are then used to establish replacement implant sizes and the alignment of bone cuts to indicate augmentation of bony defects and to confirm the anatomic joint line.

Exposure

Exposure is the key to revision knee surgery. A few minutes spent at the beginning of the surgery to get good exposure will save the surgeon from struggling throughout the remainder of the procedure. The concepts of “minimally invasive” or “mini-incision” do not apply in the revision setting. Adequate exposure aids in the removal of failed components, debridement, and assessment of bony defects and in the eventual reimplantation of revision components.

When possible, the scar from the primary procedure is followed. Where parallel incisions are present, the more lateral is usually preferred, because the blood supply to the extensor surface is medially dominant. Where a transverse patellectomy scar is present, the incision should transect it at 90°. Where there are multiple incision scars or substantial cutaneous damage (patients with burns or skin grafts, etc), one may decide to consult a plastic surgeon before surgery to design the incision, determine the efficacy of preoperative soft tissue expansion, and plan for appropriate soft-tissue coverage at closure.

Where patellar mobilization difficulties persist, a quadriceps snip, a proximal inverted quadriceps incision (modified V-Y), or a tibial-tubercle osteotomy may be indicated.¹⁴ Appropriate ligamentous release is performed based on preoperative and intraoperative evaluation. Fibrous adhesions are released to reestablish the suprapatellar pouch and medial and lateral gutters. In many revision patients, the posterior cruciate ligament will be absent or nonfunctional; any residual portion is excised. Intra-operative cultures are obtained at every revision.

Extraction of Implants

Care is taken to preserve as much bone as possible. To this end, a selection of tools is assembled, including thin osteotomes, an oscillating saw, a Gigli saw, a high-speed burr, and various extraction devices, although in many cases only the osteotome is required.¹⁵ The bone/cement or bone/prosthesis interface is carefully disrupted before extraction is attempted. The implanted components are disengaged and extracted as gently as possible to avoid fracture and unnecessary sacrifice of bone stock. When the entire prosthesis is to be replaced, it is advantageous to remove the femoral component first, because this will enhance access to the proximal tibia. All residual methyl methacrylate is cleared with chisels or power tools.

Tibial Preparation

For purposes of alignment, we recommend that the proximal tibial be prepared with reference to the position of an intramedullary (IM) rod. The knee is placed in maximum flexion with the patella laterally displaced and the tibia anteriorly subluxed (Figure 7). The location of the medullary canal is approximated, and the medullary canal is sequentially reamed with progressively larger reamers until firm endosteal engagement is established.

If the revision surgeon opts not to use a tibial metaphyseal sleeve, the proximal tibia is prepared using the metaphyseal bone tapered reamer coupled to the appropriate-sized stem trial. Once this is in place, the 2° tibial cutting block is placed, the reamer removed, and the proximal tibia resected. However, in the vast majority of our revision knee surgeries, we use a tibial metaphyseal sleeve and base the resection of the proximal tibial resection off the sleeve broach.

The mobile-bearing tibial revision broach handle is attached to the smallest broach and the appropriate trial stem (Figure 8). The broach is carefully impacted into the tibia until the top surface of the broach is at the desired proximal tibial resection level (Figure 9). If the broach is unstable or if any defect is unfilled, the procedure is repeated with consecutively larger broaches until the desired fit is achieved. It should be noted that with the mobile-bearing tibial revision tray, only the 29-mm metaphyseal sleeve can accept tibial augmentation. The broach handle is removed, leaving the last broach in place. The proximal tibia may now be resected using the top of the broach as a guide (Figure 10). The trial tibial base plate is assembled with the appropriate-sized trial metaphyseal sleeve and trial stem and inserted.

Figure 7: Adequate exposure is an essential component of revision knees surgery. The tibia is subluxed anteriorly for reaming of the medullary canal.	Figure 8: The M.B.T. Metaphyseal broach. Figure 9: Broaching for the metaphyseal sleeve.

Joint Space Assessment

The joint space is evaluated with spacer blocks to determine the flexion-extension gaps. The balance and symmetry of both the flexion and extension gaps are established as well as what prosthetic augmentation is needed to ensure postoperative equivalence (Figure 11). With the tibia sized and the approximate joint line established, the preliminary femoral component size can be selected by evaluating the explanted component, radiography templates, femoral sizing templates, femoral sizing caliper, or by sizing against the cutting guide. The femoral component size is best estimated by preoperatively templating a lateral radiograph of the operative knee before surgery or the contralateral native knee. If the contralateral knee has been replaced, a lateral radiograph is still beneficial in templating, assuming the knee is functioning well. At the time of revision surgery, the femoral component is often the same size of the tibia, or, more commonly, one size larger.

Figure 10: Proximal tibial resection using the metaphyseal broach as a guide.	Figure 11: Spacer blocks are used to determine the flexion and extension gaps and to ensure a quadrilateral space.

To decrease the flexion gap without affecting the extension gap, a larger femoral component is applied with the addition of posterior augmentation. This is particularly important where an IM stem extension is indicated, because the stem extension will determine the anteroposterior positioning of the component and the consequent flexion gap. The alternative – additional distal femoral resection and use of a thicker tibial insert to tighten the flexion gap – is not recommended, because considerable bone stock has been sacrificed in the primary procedure, and it is important that additional resection of the distal femur be avoided.

To decrease the extension gap without affecting the flexion gap, the distal femur is augmented. It is important to note that this will lower the joint line, which is usually desirable because it is generally found to be elevated in knee revision patients. This will lessen the incidence of postoperative patellar infera.

Femoral Preparation

The midline of the femoral trochlea is identified, and the medullary canal is entered with a 9-mm drill to a depth of 3-5 cm. The medullary canal is opened sequentially with reamers of progressively larger size until firm endosteal engagement is established. The flexion gap is determined next, as in the LCS Complete primary knee system. The IM rod and sleeve guide corresponding to the final reamer size are inserted into the distal femur. The anteroposterior cutting block is placed over the rod and sleeve guide, and rotation is set.

For femoral rotation, the tibial trial functions as a reference point. In a knee with competent collateral ligaments, the femoral positioner that references the tibial plateau will create a quadrilateral flexion gap (Figure 12). We find that other anatomic landmarks, such as the epicondylar axis, are unreliable. Anterior resection is performed through the anterior slot. Posterior resection is performed through the slot designated zero or, where there is posterior condyle deficiency, the appropriate 5-mm or 10-mm slot is used to accommodate posterior augments (Figure 13).

Figure 12: Femoral rotation is set forming a rectangular flexion gap.	Figure 13: Anterior and posterior resection.

Figure 14: Distal femoral resection.

The distal femoral cutting block is now assembled onto the anterior-posterior cutting block. The cutting block has slots to allow for a 0-mm clean-up cut and 5 or 10-mm distal augment. The anteroposterior cutting block and IM rod assembly are subsequently removed. In most cases, little if any bone is removed from the distal femur, because the joint line is effectively elevated with the removal of the primary femoral component. As the level of the resection is predicated on the preservation of bone stock, each condyle is cut only to the level required to establish a viable surface, with augmentation used to correct imbalance. Resection is performed through the slot appropriate for each condyle (Figure 14).

Final femoral preparation involves notch and chamfer resection. Where augmentation is planned, the appropriate augment buttons are inserted into their receptacles on the finishing guide (Figure 15). Fixation pins are introduced to secure the cutting block. The IM guide is carefully removed, and chamfer and notch cuts are made.

Optional femoral metaphyseal sleeves are also available to the knee revision surgeon. These are useful when severe bone loss is encountered in the femoral notch. Much like tibial metaphyseal sleeves, they allow for the filling of bony defects and give the femoral component a stable base for solid fixation to the metaphyseal bone. The femur is sequentially broached to the desired dimension. Care is taken to ensure that the broach remains centered in the path of the IM reamers. This will keep the metaphyseal sleeve in the desired anteroposterior position.

The trial femoral component is now assembled and inserted. Once the appropriate thickness polyethylene trial insert is in place, the knee is taken through a range of motion to verify function and stability. If the surgeon chooses, and bone stock permits, the patella may be revised as well. At this point, the trial components are removed and the final implants may be inserted.

Conclusion

The long-term clinical success of the LCS Knee Revision System as a primary knee arthroplasty system is well established. The mobile-bearing knee prosthesis was developed to restore normal knee function and to protect the polyethylene bearing from excessive forces, thereby limiting wear. Mobile-bearing prostheses are a means to achieve high conformity between the articular surfaces of the implant while maintaining acceptable polyethylene stresses at a safe level. If the knee surgeon accepts the concept of low contact stress arthroplasty, it is only logical to apply these principles to the clinically compromised patient. The LCS Complete Knee System for revision was designed to combine the established benefits of a rotating platform with a comprehensive array of implants and augments to address the issues of contemporary revision knee arthroplasty.

Figure 15: Finishing cuts are made with all augments in place.

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Authors

Drs Lippe and Crossett are from the University of Pittsburgh School of Medicine, Pittsburgh, Pa.