14Failure in Cam-Post in Total Knee Arthroplasty 14

Summary

Ideally, a total knee arthroplasty would mimic normal knee motion. Fluoroscopic studies have suggested a need for “guided motion” to provide a better approximation of normal knee kinematics. The development of the post- cam mechanism in posterior cruciate ligament-sacrific- ing designs of total knee arthroplasty is an attempt in this direction. Analysis has revealed that all post-cam mech- anisms are not the same and that substantial differences exist from one implant type to another. Mikulak et al.

have suggested that a substantial number of post-cam implants do not engage as designed.They have also raised the concept of rotational constraint leading to tibial post wear and the transmission of tibial rotational stresses to the modular interfaces (resulting in backside wear) and to the bone-implant interfaces (potentially resulting in loosening). The Puloski et al. study has also emphasized that post-cam mechanisms might result in increased wear debris and hence in negative outcomes such as osteolysis, aseptic loosening, and reactive synovitis. This group found more wear with varus/valgus constrained implants and advised caution with this design.They also described two tibial polyethylene posts that either fractured or were worn away,resulting in an unstable knee that required re- vision. This indicates that a post-cam mechanism cannot substitute for proper implant alignment and soft-tissue balancing. As we look into the future, the designers of future knee replacements that rely on a post-cam mech- anism for guided motion within a total knee replacement must be aware of the surgical and design factors neces- sary to make this mechanism function properly.

Introduction

Whether or not to sacrifice the posterior cruciate liga- ment (PCL) during primary total knee replacement remains a subject of controversy. Excellent long-term clinical outcomes have been achieved with both PCL- retaining and -sacrificing total knee arthroplasties [2-6, 9, 12-14]. Many surgeons prefer PCL-sacrificing devices when treating patients with deformities greater than 15°,

with inflammatory rheumatoid arthritis, with a prior high tibial valgus osteotomy,or with a prior patellectomy.

Recent fluoroscopic studies have also played a role in en- couraging an increasing number of surgeons to consider PCL sacrifice during total knee arthroplasty [15]. These fluoroscopic studies have suggested a rather haphazard tibiofemoral contact pattern with PCL-retaining implants and a more guided motion pattern with PCL-sacrificing devices.

Should the surgeon select a PCL-sacrificing total knee arthroplasty, options to prevent anteroposterior instabil- ity are limited to the use of a posterior-stabilized design with a tibial post and femoral cam or a dished polyethyl- ene insert with a raised anterior lip. A short-term, randomized clinical trial comparing these two devices has shown comparable short-term clinical outcomes [7].

The deep-dished, PCL-sacrificing design has the advan- tage of not relying on a small post to control anteropos- terior stability, but the disadvantage of not allowing roll-back during knee flexion, which might limit knee range of motion and the mechanical advantage of various muscles about the knee joint.

Wear of the polyethylene insert is the primary limit- ing factor in the longevity of current total knee arthro- plasties [1, 2, 10, 11, 16]. Recently, increased attention has been directed towards the post-and-cam type of PCL- sacrificing knee arthroplasties as a potential source of polyethylene damage and wear debris. The purpose of this chapter is to alert the orthopedic surgeon to the fact that cam-post PCL-sacrificing devices can be an addi- tional source of polyethylene wear debris and that there is variability in the wear patterns observed among differ- ent PCL-sacrificing designs due to differences in the cam-post mechanics, post location, and post geometry.

The surgeon should be aware that the cam-post interface is not an innocuous articulation.

Retrieval Studies

Puloski et al. retrieved and analyzed 23 post-cam, PCL- sacrificing tibial inserts over a 2-year period (1996-1998) from the practices of four orthopedic surgeons operating

14 Failure in Cam-Post in Total Knee Arthroplasty

R. B. Bourne, J.V. Baré

at his center [11]. The retrieved implants had been insert- ed over a period of 17 years (1981-1998) and had an aver- age implantation duration of 36 months (2-107 months).

Seven inserts had been implanted as primary compo- nents and 16 had been inserted during revision knee arthroplasties. The tibial inserts collected were parts of modular cemented systems from four different manufac- turers. There were four Genesis I and three Genesis II inserts (Smith & Nephew, Inc., Memphis, Tennessee), one Kinematic I, one Kinematic II, and five Kinemax inserts (Stryker Osteonics Howmedica, East Rutherford, N.J.), two AMK and five Coordinate inserts (Johnson & John- son, Depuy, Warsaw, IN), and one Insall-Burstein II and one CCK insert (Zimmer, Warsaw, IN). Eighteen inserts were manufactured from standard ultra-high-molecular- weight polyethylene (UHMWPE) and five were manufac- tured from Hylamer (Johnson and Johnston DePuy, Warsaw IN). Five of the Kinematic/Kinemax components were heat-pressed. The polyethylene sterilization proce- dure was identified for 21 inserts (gamma irradiation in air - 13, ethylene oxide - 6, and gas plasma - 2). Regarding the revision implants, six were revised for aseptic loosen- ing, one for osteolysis, seven for infection, two for pain, five for instability, and one for stiffness.

All 23 inserts were examined for wear on the stabiliz- ing post,using stereomicroscopy and digital image analy- ses. A wear score was applied to each post to calculate a

“wear grade” and “surface area” affected (

).

Initial examination of all 23 retrieved implants re- vealed evidence of wear in varying degrees over some portion of the post surface.All three mechanisms of wear occurred and the morphology of damage included pit- ting,delamination,scratching,abrasions,burnishing,and surface deformation. There was no notable evidence of embedded cement debris. The mean prevalence of all wear as a percentage of the posts’ total surface area was 39.9% (range,18.5%-60%).The most predominant mech- anism observed was adhesive wear or “burnishing”, occurring on 21 of the 23 implants (93%) and covering 24% (range, 0-48%) of the entire post surface. Fatigue

wear representing mainly delamination was evident on eight implants (36%), covering 11% (range, 0-64%) of the overall surface area. Abrasive damage involving primari- ly “scratching”was evident on 12 implants (55%),covering a surface area of 4% (range, 0-19%).

Severe post wear, involving gross loss of polyethylene was apparent in seven of the 23 inserts (30%), primarily in the form of delamination (

). Three of these seven inserts had been heat-pressed. The structural integrity of the post and capacity to prevent posterior subluxation was impaired in two implants. One of these implants, a revision Genesis I (Smith & Nephew, Memphis, TN) removed for instability after 35 months, had a fractured post that had completely dissociated from the tibial insert (

).

A Kinemax (Howmedica, Rutherford, NJ) insert, also revised for instability,had complete disintegration of 4.1 mm, or 33% of the superior apex of the post (

). Al-

⊡ Table 14-1.Wear grading system

Wear mechanism Damage mode Score Severity Add Overall

grade

Adhesive wear Burnishing 1 Mild- surface finish altered 0 1

Mod – complete loss of machining grain 1 2

Severe – palpable step around affected region 2 3

Abrasive wear Scratching 2 Mild – damage density <1/3 surface 0 2

Abrasions Mod – damage density <2/3 surface 1 3

Embedded debris Severe – damage density >2/3 surface 2 4

Fatigue wear Delamination, pitting 3 Mild – subsurface changes, surface intact 0 3

Mod – surface removed, subsurface exposed 1 4 Severe – notable wear over exposed subsurface 2 5

⊡ Fig. 14-1.Posterior surface of post from Kinematic (Howmedica, Rutherford, NJ) implant. Delamination covers over 60% of the surface.

The implant was revised for aseptic loosening after 107 months

Post wear scores (PWS) from 22 of the 23 retrieved inserts were determined.Wear analysis from one compo- nent was inapplicable, as iatrogenic damage during removal was too extensive for accurate quantitative assessment. A mean total wear score of 3.56 (±3.18, 0.7- 12.4) was calculated for all components with an average implantation period of 35.7 months (range,2.3-107).PWS, period of implantation, and revision diagnosis for all implants are shown in

.

The Pearson product-moment correlation coefficient comparing period of implantation with PWS was 0.61, suggesting an increasing wear score with duration (

). With the numbers available, no significant differences in normalized wear scores were noted be-

14

though formal grade analysis was not completed on the articular surfaces of the tibial inserts in this study, severe wear of the tibial post did appear to coincide with appre- ciable wear over the articular surface. Certainly, delami- nation occurring over the post surface seemed to corre- spond with similar damage over the articular surface in five of the seven severely damaged inserts. Interestingly, a single AMK insert, revised for pain after 45 months duration, exhibited severe wear over a portion of the anterior and posterior post surface without evidence of similar articular surface wear (

). Intra-opera- tive records made during explantation of this component describe an inflammatory cyst and generalized reactive synovitis with appreciable evidence of polyethylene debris.

⊡ Fig. 14-2.Genesis (Smith & Nephew, Memphis, TN) tibial insert revised for instability after 35 months. The stabilizing bar was found to be frac- tured at the time of revision

⊡ Fig. 14-3. Anterior surface of stabilizing post from Kinematic (Howmedica Rutherford, NJ) tibial insert. This implant exhibited cata- strophic wear with severe loss of polyethylene. Over 30% of superior apex of the post is worn away. This implant was revised after 77 months due to instability

⊡ Fig. 14-4.Anterior surface of stabilizing post from AMK (DePuy, War- saw, IN) insert. Significant post wear with minimal condylar surface wear was observed in this component. The implant was revised after 45 months for pain from a chronic synovitis

Post Wear Sorce

14 12 10 8 6 4 2

00 20 40 60 80 100

Period of Implantation(mo)

⊡ Fig. 14-5.Post wear score and period of implantation for all implants.

Pearson product-moment correlation coefficient, r=0.61

tween design types.Similarly,no significant differences in normalized post wear scores were noted when comparing inserts manufactured from Hylamer or standard UHMWPE, or between sterilization procedures. The nor- malized PWS for heat-pressed inserts were appreciably higher than the non-heat-pressed inserts, but with the small numbers available,no significance was determined.

Average wear score and period of implantation in components revised for infection was 1.56 (±0.65), asep- tic loosening was 3.7 (±1.48), osteolysis was 12.4; pain was

2.29 (±1.96), instability was 4.5 (±3.98), and recurrent effusions was 7.2.With the small sample size and absence of complete tibial surface analysis, significance of testing of post-wear score and revision diagnosis would be irrel- evant.It was noted,however,that implants revised for sep- sis did exhibit normalized wear scores and wear patterns similar to those of the rest of the cohort.Fatigue wear was not observed over the post in any of the implants revised for infection, but the average duration was relatively short; 16.5 (range, 2.3-31.3).

⊡ Table 14-2.Implant demographics and tibial post wear scores

Implant Model Manufacturer Material Sterilization Period Revision diagnosis Post wear

(months) score (PWS)

1 Kinemax Howmedica UHMWPE Gamma 33.6 Aseptic loosening N/A

2 Kinemax Howmedica UHMWPE Gamma 5.8 Instability 1.2

3 Kinemax Howmedica UHMWPE Gamma 25.0 Infection 2.1

4 Kinemax Howmedica UHMWPE Gamma 107.2 Aseptic loosening 5.6

5 Kinemax Howmedica UHMWPE Gamma 77.1 Instability 8.0

6 Kinematic II Howmedica UHMWPE Gamma 53.0 Osteolysis 12.4

7 Kinematic I Howmedica UHMWPE Gamma 103.3 Aseptic loosening 4.5

8 CCK Zimmer UHMWPE N/A 18.4 Aseptic loosening 3.6

9 IB II Zimmer UHMWPE N/A 17.7 Infection 1.4

10 Genesis II Smith & Nephew UHMWPE ETO 3.5 Stiffness 0.8

11 Genesis II Smith & Nephew UHMWPE ETO 23.6 Pain 1.6

12 Genesis II Smith & Nephew UHMWPE ETO 2.3 Infection 0.7

13 Genesis Smith & Nephew UHMWPE ETO 31.3 Infection 2.2

14 Genesis Smith & Nephew UHMWPE Gamma 84.8 Recurrent effusion 7.3

15 Genesis Smith & Nephew UHMWPE Gamma 35.4 Instability 9.6

16 Genesis Smith & Nephew UHMWPE ETO 30.7 Aseptic loosening 2.7

17 AMK Depuy UHMWPE Gas plasma 9.8 Infection 1.2

18 AMK Depuy UHMWPE Gamma 45.3 Pain 4.5

19 Coordinate Depuy Hylamer Gas plasma 21.2 Infection 1.0

20 Coordinate Depuy Hylamer Gamma 8.0 Infection 2.2

21 Coordinate Depuy Hylamer Gamma 24.7 Aseptic loosening 1.9

22 Coordinate Depuy Hylamer Gamma 13.1 Instability 1.4

23 Coordinate Depuy Hylamer Gamma 44.1 Instability 2.4

Wear Score/Period of Implantation (mo)

0.12

0.1

0.08

0.06

0.04

0.02

0

A II (22) Kinematic Kinemax (6)

Genesis I/II (7) AMK Coordinate (7)

IB II/CCK (2) Anterior Posterior Medial Lateral

⊡ Fig. 14-6.Mean normalized wear scores by surface location combined for all implants and for individual de- signs. Number of implants per group in parentheses

14-6

).The most predominant location of wear and highest wear scores occurred over the posterior surface, prevalent in all 22 (100%) of the inserts with a mean wear score of 1.86 (±1.53). The incidence of anterior wear was not in- significant, occurring in eight (36%) of the implants with a mean wear score of 0.93 (±1.56).The overall extent of an- terior wear was skewed by the high incidence and severi- ty of wear in the group of Kinematic/Kinemax (Howmed- ica, Rutherford, NJ) inserts.All six of these inserts (100%) had evidence of anterior wear and exhibited significantly (p<0.05) higher normalized wear scores over the anterior surface when compared with the other designs.

With the numbers available, no significant difference between medial and lateral post wear was identified. The predominant morphology of wear over the medial and lateral surfaces was burnishing, with minimal evidence of fatigue. Posts with a relatively wider medial-lateral dimension had increased evidence of damage over these surfaces. This was evident in six implants, five Co-ordi- nate (DePuy,Warsaw,IN) and one CCK (Zimmer,Warsaw, IN), which had ultra-constraining posts used to resist varus-valgus deformity and to limit tibial rotation to a few degrees.Normalized wear scores were substantially high- er over the medial and lateral surfaces of these con- strained posts when compared with wear scores over the same regions from implants of a similar model type (

). This is evident in the relatively high mean scores in the two groups containing these implants (see Fig. 14-6). In two cases, significant wear of either the me- dial or lateral articular surface appeared to have caused a mild varus-valgus deformity, which appeared to acceler- ate wear over the opposite surface of the post.

Wear occurred over all four surfaces of the post, but the prevalence varied among implant designs. The most predominant location of wear was the posterior surface, and this was found to be worn in all 22 inserts. Anterior

morphology was burnishing. Medial-lateral wear was worse in the six varus/valgus constrained implants.

We can conclude that PCL-sacrificing post-cam de- vices may contribute to the production of additional wear debris and be associated with negative outcomes such as reactive synovitis, aseptic loosening, and osteolysis. The surgeon should be aware that the cam-post interface is not an innocuous articulation. Manufacturers should strive to limit wear and damage to these components.

Finally, highly congruent varus/valgus designs demon- strate increased wear and should be used with caution.

Clinical Studies

Mikulak et al. analyzed 557 consecutive press-fit condylar modular knee prostheses with a PCL-substituting design (PFC, Johnson & Johnson Orthopaedics, Raynham, MA) implanted by a single surgeon between July 20, 1989 and October 14, 1994 [10]. All knees were inserted according to a standard protocol. At a mean of 56 months (37-89 months) postoperatively, 22 (3.9%) of the 557 primary knee arthroplasties had been revised. Four (0.7%) were revised because of infection, two (0.4%) because of a supracondylar fracture, and 16 (2.9%) because of aseptic loosening and osteolysis.

The authors went on to provide a detailed description of the 16 knee replacements (15 patients) which had been revised because of aseptic loosening. The study popula- tion of failed total knee arthroplasties was made up of eight women and seven men with a mean age of 65 years (46-80 years). All patients were doing well at 1 year, with Knee Society clinical and functional scores of 95 and 86 points, respectively. These scores declined to 48 and 45 points, respectively, prior to revision. Six patients had isolated loosening of the femoral component with oste- olysis, six had loosening of both the femoral and tibial components with osteolysis, and two had isolated loos- ening of the tibial component with osteolysis.At retrieval, the machine marks on the posterior aspect of the poly- ethylene post were preserved in six of the 12 retrieved polyethylene inserts, indicating a lack of substantial repetitive contact between the tibal post and the femoral cam of the components. All 12 inserts had burnishing on both the medial and lateral aspects of the tibial post. In ten of the 12 knees, grooves of variable depth associated with burnishing and delamination were found at the an- teromedial and anterolateral edges of the post. A hori- zontal burnished groove across the anteroinferior aspect of the polyethylene post was seen in nine of the 12 inserts.

With as little as a 1-mm reduction in the thickness of the tibial condylar polyethylene, the top of the tibial post impinged on the roof of the femoral box housing. An

14

Wear Scorel Period of Implantation (mo)

0.06

0.05

0.04

0.03

0.02

0.01

0 Constrained Unconstrained

Medial Lateral

⊡ Fig. 14-7.Normalized mean wear scores from medial and lateral sur- faces of constrained (CCK, Co-ordinate) and unconstrained (AMK, Insall- Burstein II) inserts of same manufacturer and similar modular design

asymmetric loss of polyethylene from the top of the post reflected this impingement. Overall, patient activity, younger age, and greater height were correlated with increased wear on the medial side of the tibial insert.

So-called backside wear was seen on all 12 of the tibial polyethylene inserts.

In summary, this clinical study made several interest- ing observations with regard to the post-cam mechanism in this particular PCL-substituting total knee arthro- plasty. Six of the 12 retrieved tibial components demon- strated little evidence of cam-post engagement, despite a good range of motion. When the cam-post mechanism did engage, as suggested by wear on the posterior aspect of the tibial polyethylene post, none of these patients revealed any type of surface damage on the articulating or back sides of the tibial insert, suggesting that when the cam-post mechanism works, the vector sum of the tibial femoral forces and the cam-post forces are compression into the tibia. The authors also found evidence of surface damage to the medial and lateral sides and base of all posts retrieved. With the design of total knee replace- ments studied, the authors concluded that rotational forces generated by impingement of the side walls of the intracondylar box on the polyethylene post produced sur- face damage. The authors also demonstrated that the top of the polyethylene post could impinge in the roof of the box in the femoral component if even minimal wear oc- curred in the polyethylene insert. The authors ended this study by implicating rotational constraint in post-cam types of PCL-substituting total knee arthroplasties as a cause of post wear and the transmission of rotational stresses to the modular interfaces resulting in wear into the bone-implant interfaces, potentially resulting in loos- ening.

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