35Optimizing Cementing Technique 35 35

Chapter 35 · Optimizing Cementing Technique – G.R. Scuderi, H. Clarke

35 Optimizing Cementing Technique

G. R. Scuderi, H. Clarke

Summary

Supported by the long-term clinical success and sur- vivorship analysis,cemented total knee arthroplasty con- tinues to be the gold standard against which alternative means of fixation need to be compared. Fundamental to implant longevity is meticulous technique, bone prepa- ration, and handling of the cement. Fixation of poly- methylmethacrylate to the cancellous bony surface is achieved by the irregular configuration of the bony sur- face and the penetration of the cement into the bone.

Well-fixed cemented components have shown very little micromotion at the fixation interface,with little displace- ment over the years. A well-designed and properly posi- tioned cemented total knee arthroplasty has a greater than 90% chance of surviving more than 15 years.

Introduction

Supported by the long-term clinical success and survivor- ship analysis, cemented total knee arthroplasty continues to be the gold standard against which alternative means of fixation need to be compared. A well-designed and prop- erly positioned cemented total knee replacement has a greater than 90% chance of surviving more than 15 years [11] (

).This success consistently surpasses the re- sults of cementless total knee replacement [9].

Polymethylmethacrylate (PMMA) is a derivative of acrylic acid that is formed by the combination of a monomer liquid mixed with a polymer powder that leads to an exothermic reaction as it changes into a solid state.

All cements are not identical.The polymerization process takes several minutes with the change from the liquid state, through a doughy period, and into a solid material [30].The liquid or wetting stage is usually short,while the doughy stage is more variable and susceptible to outside factors such as ambient temperature or humidity in the operating room. The final stage to a solid state is not sig- nificantly influenced by outside factors, but may vary from one brand to another.

Bone preparation is critical for adequate penetration of the cement into the cancellous surface. PMMA acts as

an intermediary between the prosthesis and the bone.

The cement is not an adhesive agent and performs best under compressive loads [30].

Cement Technique

Fundamental to implant longevity is meticulous tech- nique,bone preparation and handling of the cement [26].

Fixation of PMMA to the cancellous bony surface is achieved by the porosity of the bone and the penetration of the cement into the bone. Since bone penetration is critical to the intrusion of PMMA into the cancellous bone, the resected bone surfaces are cleansed with pul- satile irrigation to remove blood, fat, and bone debris.

Proper preparation allows for uninhibited penetration of the cement into the bone.Our preference is to use Simplex cement (Howmedica,Rutherford,NJ) in a doughy tactless state. This permits easy handling and manual pressuriza- tion of the cement into the porous bone.The ideal cement penetration into the bone is 1-2 mm. With soft rheuma- toid bone, deeper cement penetration may occur. On the other hand, with hard sclerotic bone, the bone surface

⊡ Fig. 35-1.The AP radi- ograph demonstrates a pris- tine cement mantle at 15 years

should be drilled or abraded to allow the cement to grasp the bone surface.

This cement technique has not changed for over two decades [25]. The components are cemented in a sequen- tial fashion,and we choose to mix two separate batches of cement. With the first batch of cement, an all polyethyl- ene patellar component along with the femoral compo- nent is cemented in place. While the patellar component is held in place with the patellar clamp, a small amount of cement is placed on the posterior condyles of the femoral component (

). The remainder of the cement is placed in a horseshoe-shaped fashion over the anterior and distal surface of the prepared femur (

). The femoral component is then impacted in place and the ex- cessive cement is removed.The precise femoral cuts allow a tight fit between the bone and prosthesis such that the resultant cement mantle is approximately 1 mm thick.The tibial component is then cemented in place with the sec- ond batch of cement. The entire tibial component, in- cluding the central stem, is cemented in place (

).All excess cement must be removed from around the components to prevent cement particles from breaking

loose (

). The presence of entrapped cement be- tween the articular surfaces leads to third-body wear and damage to the polyethylene. The postoperative radi- ograph should demonstrate a perfect cement mantle around the components (

).

The importance of initial prosthetic fixation has been emphasized in the past,since it has been shown that pros- theses that continuously migrate will eventually loosen.

The most decisive time is during the operative procedure and influenced by the surgical technique and prosthetic design. While cementing the femoral component has been standardized, there appears to be some variation to cementing the tibial component. Some tibial tray designs have a cruciate-shaped central stem, which allows “hy- brid” fixation [23]. This means that the undersurface of the tray is cemented while the stem is press-fit into the metaphyseal bone.This is design specific,since other cen- tral stem designs have an I-beam configuration, which should be completely cemented. Prosthetic designs such as the Total Condylar and Posterior Stabilized Prostheses (Zimmer, Warsaw, IN) were designed for routinely ce- menting the entire tibial component including the central stem [23].In order to do so,cement is pushed into the cen- tral stem hole in the tibia and the cement is manually pressurized on the tibial plateau. It has always been our

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⊡ Fig. 35-2.Cement is placed on the posterior condyles of the femoral component prior to impaction

⊡ Fig. 35-3. The doughy cement is placed in a “horseshoe” fashion around the distal femur

a

⊡ Fig. 35-4a, b.Since the entire tibial component is cemented, cement is placed on the proximal tibial surface (a) as well as pushed into the cen- tral fixation hole (b)

b

recommendation that the tibial component be complete- ly cemented. “Hybrid” or surface tibial cement fixation has little merit and is prone to unacceptable rates of loos- ening. Gunderson et al. have shown a 9% tibial loosening rate with surface cementation compared with no tibial component loosening with a fully cemented tibial com- ponent [13].This result led those investigators to abandon the surface cementation technique. Bert further supports cemented stems with an in vitro study showing that a 1- mm cement mantle surrounding the central stem im- proves stability of the tibial component [4]. Additionally, Ryd [21] and Albrektsson [2] have shown that,when com- pared with cementless fixation, the addition of cement to the implant has reduced micromotion. This reduction in micromotion greatly influences the final outcome be- cause if there is little inducible displacement of a pros- thesis at 6 weeks, there will be little inducible displace- ment after 1 year and little migration after 2 years [26].

Modes of Cement Failure

Though component loosening was the most frequent re- ported cause of failure with early designs,it is not accurate to blame it entirely on cement fixation.Early prosthetic de- signs with limited sizes and instrumentation did not allow the kinematics, alignment, and soft-tissue balancing to be optimized. The original linked prostheses were highly constrained, placing stresses on the bone-cement inter- faces. This resulted in high rates of loosening [12]. The in- troduction of less-constrained surface-replacing cement- ed prostheses, designed with greater attention to knee kinematics, seems to have resolved some of the earlier problems.Despite the better than expected results with ce- mented TKR, cases of aseptic loosening do occur. There are those who believe that micromotion at the bone-ce- ment interface progresses to macromotion with eventual bone loss and component loosening, while others specu- late that the underlying bone, when subjected to uneven stresses,subsides,leading to component loosening.This is particularly the case with varus malalignment of compo- nents. Subsidence is a problem of surgical technique and the underlying cancellous bone, not of cement fixation.

Concerns about constraint and increased stress at the bone-cement interface have been a recurring issue in the debate about PCL retention versus substitution in TKA.

Advocates of PCL retention are concerned that the in- creased constraint with PCL-substituting designs results in increased stresses on the bone-cement interface. In fact, the interaction of the femoral cam and tibial spine with the Insall Burstein Posterior Stabilized Knee (Zim- mer,Warsaw,IN.),imparts a compressive force on the tib- ia that negates tibial component liftoff [14]. Furthermore, metal backing of the tibial component has been shown to transmit the load better to the underlying bone [3].

Chapter 35 · Optimizing Cementing Technique – G.R. Scuderi, H. Clarke

⊡ Fig. 35-5a, b.With the components fully seated, all excess cement is removed from around the femoral (a) and tibial (b) components in an ef- fort to prevent particles from breaking free and getting trapped in the ar- ticulation

a

b

⊡ Fig. 35-6a, b. The anteroposterior (a) and lateral (b) radiographs demonstrate an ideal cement mantle

a b

The clinical success of cemented TKR supports its con- tinued use. Posterior cruciate-retaining designs such as the kinematic total knee prosthesis (Howmedica,Ruther- ford,NJ) have had long-term success [10].In a 5- to 9-year follow-up study of this prosthetic design,the investigators have reported 90% good or excellent results. Though there were eight patellar complications in this study,there were no loose femoral or tibial components. Similarly, in a review of his last 1000 consecutive primary TKRs with a PCL-retaining design, Scott had no femoral or tibial components loosen [23].

The total condylar prosthesis,which sacrificed the PCL, was one of the first modern cemented knee prosthesis.

Along with Ranawat [18],Vince et al. [31] have reported ex- cellent results with this prosthetic design, supporting the belief that cemented TKR is a durable and predictable pro- cedure.Despite the success of the total condylar prosthesis, the posterior-stabilized prosthesis was introduced [14].The intent was to design a prosthesis that improved stair-climb- ing, increased range of motion, and prevented tibial sub- luxation. The early and midterm results were very favor- able. Aglietti and Buzzi [1] reported 90% good and excel- lent results at 3- to 8 year follow-up. Scott and co-workers [24] demonstrated 98% excellent and good results at 2- to 8-years. In 1992, Stern and Insall [27] reported on the long- term results of the posterior-stabilized prosthesis.The 9- to 12-year results with an all-polyethylene tibial component produced 87% good and excellent results,which were com- parable to the long-term results with the total condylar prosthesis. Analysis of the failures in the posterior stabi- lized series shows that there were five infections,three loose femoral components (1.5%), and six loose tibial compo- nents (3%). Metal backing of the tibial component im- proved fixation. In a 10- to 12-year follow-up study of the posterior-stabilized prosthesis with a metal-backed tibial component Colizza et al. [6] reported 96% excellent and good results. Despite the occurrence of two loose femoral components, there were no loose tibial components. The remaining two failures included one knee revised for re- current hemarthosis of unknown etiology and another for postoperative recurvatum. In both of these cases, the ce- mented components were well fixed.These results confirm the advantage of prosthetic conformity in minimizing polyethylene wear without compromising fixation.

Several other cemented posterior-stabilized designs have also yielded comparable results. Ranawat reviewed the 4- to 6-year results with the modular Press Fit Condy- lar PCL-substituting design (Johnson & Johnson,NJ) [18].

He reported 93% excellent and good results with no cas- es of component loosening.

Speculating that the level of activity would influence the longevity of cemented TKR, Diduch et.al. evaluated the long-term results and the functional outcome in pa-

excellent at an average follow-up of 8 years. The 18-year cumulative survivorship was 94%.This was a group of pa- tients who regularly participated in physical activities, which placed high stresses on the cement interfaces.

Though there was one case of polyethylene wear, there were no cases of component loosening.

Following the introduction of modular tibial compo- nents in 1987, there have been recent concerns that poly- ethylene wear on the backside of the tibial component would lead to osteolysis. Brassard et al. attempted to eval- uate this theoretical concern with a long -term evaluation of the modular Insall Burstein Prosthesis [5]. In this radi- ographic review, there were no cases of massive osteoly- sis, but the authors did mention that three knees had lo- cal minimally progressive lesions which were not clini- cally significant. This series of metal-backed tibial components had an overall incidence of tibial component radiolucent lines of 11% compared with 49% seen with the all-polyethylene tibial component [5, 27]. Therefore, the introduction of modularity to this particular implant de- sign did not appear to raise concerns about osteolysis.

Survivorship analysis has been a useful tool in deter- mining the durability of an implant design.This method of analysis depends on the definition of implant failure. Suc- cess is usually defined as a prosthesis that is still in place, while failure is defined as a knee that has been revised or a revision has been recommended. The 12-year cumulative success of the Insall Burstein Posterior Stabilized Prosthe- sis with an all-polyethylene tibial component is 94% [27].

In more recent long-term studies of the Insall Burstein Pos- terior Prosthesis with a metal-backed tibial component, both best- and worst-case scenarios have been tested. The best-case scenario revealed a cumulative success of 96.4%

at 11 years. In contrast, the worst-case scenario considers knees that are lost to follow-up as failures. This scenario yielded a cumulative success of 92.6% at 11 years [5, 6]. A further testimony to implant durability is a 94% survivor- ship at 18 years in an active patient population under the age of 55 years with a posterior-stabilized prosthesis [7].

Despite the long-term success of the original Insall Burstein design,incremental technical improvements and modifications have been made. These include the intro- duction of metal-backed and modular tibial components, modifications of the trochlear geometry to optimize patellofemoral kinematics and optimization of the spine- cam mechanism to reduce the risk of dislocation. There- fore, while the central principles of the Insall Burstein prosthetic design have been preserved,an evolution of this design has occurred. In the latest major redesign, the In- sall Burstein Posterior Stabilized Prosthesis evolved into the NexGen Legacy Posterior Stabilized Prosthesis (Zim- mer, Warsaw, IN) in 1997 (see Fig. 35-2). Recently we re- viewed our initial cohort of patients with this prosthesis: 233

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patients underwent 279 primary total knee arthroplasties between August 1997 and December 1999. Ten patients (ten knees) subsequently died, 16 patients (16 knees) were excluded because of severe medical disability, and 12 pa- tients (13 knees) were lost to follow-up. Thus, 195 patients (240 knees) were available for analysis. The mean age at the time of operation was 66 years. The mean duration of follow-up was 48 months (range 24-72 months).Pre-oper- atively, the mean arc of motion was 107°, compared with 117° at the latest follow-up examination.The mean pre-op- erative Knee Society Knee Score was 48 points, compared with 96 points at the latest follow-up examination. The mean Knee Society Functional Score was 83 points at the latest follow-up examination.Radiographic evaluation re- vealed an incidence of minor radiolucent lines of 4%, and their presence was of no clinical significance.There was no evidence of loosening, osteolysis or polyethylene wear.

A comparison of current designs reveals that ce- mented TKA has a longer predicted survival than ce- mentless implants. In a clinical and radiographic com- parison of cemented and cementless fixation with the Miller-Galante Prosthesis (Zimmer, Warsaw, IN.), Rosen- berg found no cemented implant that failed due to loss of fixation, while three cementless implants failed due to lack of tibial bone ingrowth [20].While an early compar- ison study of cemented and cementless Porous Coated Anatomic prostheses (Howmedica, Rutherford, NJ) demonstrated comparable results, cementless fixation has shown a precipitous decline in successful results with longer follow-up [8, 15-17].

While some surgeons may seek other means of fixa- tion, cemented TKR should be the gold standard against which alternative fixation techniques are compared. A well-designed cemented prosthesis, implanted with meticulous surgical technique, has proven to be pre- dictable and durable with excellent long-term results.

References

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23. Scott RD (1996) Posterior cruciate ligament retaining designs and results.

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