The Evolution of Modern Knee Prosthesis

History

In the 9th century pig bladder, nylon, fascia lata or even cellophane were interposed between the femur and tibia to cover up the damaged articular surfaces of the knee joint in order to lessen pain and deformity. But all failed.

In 1860 Ferguson resected the lower end of femur and the upper end of tibia to create new painless articular surfaces but the tendons and the ligaments became lax and the joint unstable. Obviously the suffering became more. One major set of discovery around 1963 ignited the hope of success.
It was the introduction of:

1. High density polyethylene and the discovery of the
2. Bone cement as a method of fixation.

Total Condylar Prosthesis (TCP) of Insall (1973)

At last, in 1973 the magic discovery of a total articular RESURFACING PROSTHESIS known as the TOTAL CONDYLAR PROSTHESIS (TCP) designed and developed by Insall paved the way for the journey of the modern total knee arthroplasty (TKA) era. It was such a big breakthrough that even almost five decades later there has been no other major technologic advancement in this field.

Insall set aside the hypothesis of reproducing anatomical kinematics of normal knee joint motion and gave more emphasis on functional and mechanical aspects. It was truly the first TKA with reliable and replicable functional outcomes and long-term survivorship. In this prosthesis, BOTH cruciate ligaments were sacrificed (but there was NO POST to replace it) and the articular surface geometry determined joint’s sagittal plane stability. The femoral condyles were symmetric.

In this way Insall’s Total Condylar Prosthesis (TCP) was actually a Bicruciate-Sacrificing Total Condylar Prosthesis. Removing the cruciate ligaments simplified the TKA kinematics.

The TCP had Chrome-Cobalt femoral, and polyethylene tibial and patellar components.

The femoral and tibial components were completely congruent in flexion and extension and there was a CENTRAL CAM which provided flexion stability. But its main problem was its high conformity and congruency which prevented femoral rollback on tibia leading to limited knee flexion, also sometimes there was anterior subluxation during flexion.

The original TCP was a totally cemented prosthesis which boasted long-term survivorship of over 90% after a 15- to 20-year of follow-up.

Insall-Burstein (IB) Total Condylar Knee (1978)

At last, in 1973 the magic discovery of a total articular RESURFACING PROSTHESIS known as the TOTAL CONDYLAR PROSTHESIS (TCP) designed and developed by Insall paved the way for the journey of the modern total knee arthroplasty (TKA) era. It was such a big breakthrough that even almost five decades later there has been no other major technologic advancement in this field.

Insall set aside the hypothesis of reproducing anatomical kinematics of normal knee joint motion and gave more emphasis on functional and mechanical aspects. It was truly the first TKA with reliable and replicable functional outcomes and long-term survivorship. In this prosthesis, BOTH cruciate ligaments were sacrificed (but there was NO POST to replace it) and the articular surface geometry determined joint’s sagittal plane stability. The femoral condyles were symmetric.

In this way Insall’s Total Condylar Prosthesis (TCP) was actually a Bicruciate-Sacrificing Total Condylar Prosthesis. Removing the cruciate ligaments simplified the TKA kinematics. The TCP had Chrome-Cobalt femoral and polyethylene tibial and patellar components.

The femoral and tibial components were completely congruent in flexion and extension and there was a CENTRAL CAM which provided flexion stability. But its main problem was its high conformity and congruency which prevented femoral rollback on tibia leading to limited knee flexion, also sometimes there was anterior subluxation during flexion.

The original TCP was a totally cemented prosthesis which boastedlong-term survivorship of over 90% after a 15- to 20-year of follow-up.

Insall-Burstein (IB) Total Condylar Knee (1978)

In 1978 came the Insall-Burstein total condylar knee, which tried to correct the limitations of the Installs’ original TCP. There was a central cam on the femur and a post on the tibia. It had a more posterior point of contact.

Due to the CAM-AND-POST MECHANISM this design of TKA reached 110° degrees of knee flexion. The cam engaged the back of tibial post at 70° of flexion and shifted the contact point posteriorly during flexion to reproduce FEMORAL ROLLBACK.

IB Knee was the first Posterior-Stabilized (PS) prosthesis. In this design medial and lateral condyles were symmetric and a decreasing sagittal radius of curvature posteriorly. Both the condyles were symmetric and convex in the coronal plane. But its major problem was an unacceptably high incidence of PATELLAR DISLOCATION. Since then an asymmetric anterior flange was created (higher laterally) to improve patellofemoral tracking.

There were two articular surfaces of the tibial polyethylene, perfectly congruent with the femoral component both in extension (in sagittal plane) and in flexion (in coronal plane). The median eminence and the anterior and posterior lips of the tibial component served resist translation and dislocation of the components. There was a METAPHYSEAL STEM in the tibial component to resist tilting of the prosthesis during asymmetric loading.

The original TCP and IB knee used all-polyethylene cemented tibial components.

Metal-backing of the tibial component was added later to allow for improved load distribution on the tibial condyles, and to protect cancellous / osteoporotic metaphyseal bone as well as the polyethylene at the cement- implant interface. The patella was fitted with a dome shaped all-polyethylene button fixed to the native patella with a central fixation lug. Even today’s modern prostheses also use many of these design features.

PCL Retaining Duopatellar Prosthesis

Alongside the development of the cruciate-sacrificing total condylar prosthesis, the Duo patellar prosthesis was invented to preserve cruciate ligaments, particularly the PCL. Its femoral component was anatomically shaped with a sagittal plane contour.

The initial design of the Duopatellar knee had two separate tibial plateau components. Later, the two components were joined to a one-piece tibial component with a cutout for the PCL. The patellar component was identical to that of the Total Condylar Prosthesis. The Duopatellar prosthesis preserves the cruciate ligaments, particularly the PCL.

Kinematic Condylar Prosthesis

The duopatellar prosthesis gave way to cemented kinematic total knee arthroplasties with a kinematic condylar prosthesis. It was widely used in the 1980s and yielded equally good long-term results with OA and RA. Deep infection and component wear were the main reasons for failure and revision.

Drawbacks of the Total Condylar Prosthesis

The early drawbacks of the total condylar prosthesis:

1. In cases where the flexion gap was larger than the extension gap it had a tendency to subluxate posteriorly in flexion.
2. In cases where the PCL was non-functioning there was no femoral rollback, and the range of flexion was less. The result was impingement of the posterior femoral metaphysis at around 95 degrees of flexion.

Due to absence of the posterior condylar support and forward femoral component rollback on tibial tray, there was abutment or edge loading on posterior lip of the femur against polyethylene insert resulting in limited flexion. But if there was proper posterior femoral offset and a healthy PCL, the impingement of the posterior femur against tibial insert could be avoided and flexion optimized. The reported average flexion of the early total condylar prosthesis was only 90 to 100 degrees.

These problems were addressed in 1978 by Insall and Burstein by adding a central cam and post mechanism to the total condylar prosthesis. This design was called as Insall-Burstein posterior cruciate-substituting (CS) or posterior-stabilized (PS) knee prosthesis. Due to the cam-and-post mechanism in this design of TKA the knee flexion reached 110 degrees.

Through this design the posterior contact point of the femur and tibia was shifted more posteriorly allowing femoral rollback and more flexion. In this design a cam on the femoral component engaged a central post on the tibia at about 70 degrees of flexion.

Most of the current TKA designs developed from the Insall-Burstein Knee and kinematic condylar designs.

Patellofemoral (PF) complications

In late 1980s and 1990s, patellofemoral complications were one of
the primary causes for revision in TKA. As a result later designs paid more attention to better reconstruction of the patellofemoral joint (PFJ). Consequently, greater areas of patellofemoral contact were incorporated.
Main improvements were twofold:

1. The length of the trochlear groove was increased which minimized soft-tissue reaction caused by a short trochlear groove, and
2. A more asymmetric anterior flanges (higher laterally) to counteract patellar subluxation thus improving patellofemoral tracking.

Many newer designs now have a deep trochlear groove and asulcus cut to the anterior chamfer of the femur.

Cruciate-retaining Knee (CR Knee)

The controversy over whether to retain or substitute for the posterior cruciate ligament (PCL) in total knee arthroplasty is still live. Three opinions exist. One exclusively preserves the PCL, the second exclusively substitutes for it, and the third is selective.

The PCL is a natural knee stabilizer in flexion. It is critical to normal knee kinematics.

The native PCL in CR-TKA design prevents anterior translation of the femur during knee flexion, and therefore there is no Cam-Post mechanism here.

Advantages of PCL Retention

1. Retention of a structure necessary for the proprioception.
2. More natural femoral rollback.
3. Preservation of a natural central stabilizer (i.e. PCL) of the joint.
4. With no post-cam mechanism in place there is less constraint and less shear stress imparted to the insert-tray interface and the prosthesis-bone interface.
5. Near-normal joint-line location.
6. More bone preservation

Numerous reports on Cruciate-Retaining (CR) Total Knee Arthroplasty have shown consistently good clinical results and excellent intermediate and long-term survivorship.

Richard D. Scott believed that “at least 98% of primary knees can be treated with PCL retention”.

The main drawback of CR-TKA is that the balancing the PCL may be difficult and tricky and the distal attachment of the PCL may get ruptured. The CR-TKA demonstrates unphysiological roll-forward of the medial femoral condyle (MFC) starting at 30 degrees of flexion as well as minimal LFC rollback when the native PCL is weak.

As a remedy, some total knee manufacturers have used a deep polyethylene insert or tray option to their CR designs for use in this situation. In this regard the CR-TKA resembles the original total condylar design which had a deep-dish polyethylene component and sagittal plane concavity or dishing to impart anterior and posterior translational stability. The resultant build-up of the anterior aspect prevents major roll forward of femoral condyles on tibial polyethylene.

On comparison, the deep-dish CR knee proved as good as the post and cam fitted PS knee using the same femoral components in terms of pain scores, range of motion (ROM) and ability to climb or descend stairs. The deep-dish design of CR knee incorporated many of the advantages of cruciate sacrifice, reducing chances of fracture due to obligatory bone loss in the intercondylar region of the femur in PS knee. But some CR designs proved not as stable as PS designs due to sagittal-plane laxity.

After proper flexion-extension gap balancing the CR knee yielded flexion similar to the PS design, by avoiding posterior impingement in flexion. ROM after PCR-TKA further improved when the posterior condylar offset is maintained. Over resection of the posterior femoral condyles results in suboptimal flexion because then the posterior aspect of tibia may abut the posterior aspect of the femur.

Important technical points for a CR knee replacement are:

(1) Right amount of posterior tibial slope and
(2) Reestablishing the posterior condylar offset as measured radiogarphically.

Some newer PS total knee designs (e.g. ANTHEM Total Knee System) have incorporated “bone conserving femoral PS box design, preserving intercondylar notch bone stock “as well as a dual-cam mechanism in which the post drives a screwhome mechanism during full extension. In such designs a transverse plane rotation pattern behaves closer to normal knee kinematics than with older PS knees.

PCL sacrificing designs (CS-TKA)

During the past 5 years there has been renewed interest in the use of deep-dish highly conforming implants with increased sagittal plane conformity (either with PCL recession or with PCL sacrifice).

The stability of the knee after CS TKA is dependent on the collateral ligaments, posterior capsule, and articular conformity without a cam or post. But the functional results especially flexion with these more conforming devices has been found to be similar to the PCL-retaining and PCL-substituting devices when compared.

Bicruciate designs

Efforts were also made to normalize knee kinematics either by substituting both the anterior cruciate ligament (ACL) and PCL or by retaining them.

Bicruciate-substituting (BCS) TKA

The new system of bicruciate-substituted design has TWO CAM-POST MECHANISMS, one for the ACL and another for the PCL. It is postulated that the added constraints will guide the motion of the knee during flexion and extension. But this knee system did not result in improved knee flexion or better clinical and functional outcomes than their conventional counterparts.

Bicruciate-retaining (BCR) TKA

Efforts for such design date back to the 1980s. Bicruciate-retaining (BCR) TKA intended to mimic natural knee biomechanics through ACL as well as PCL preservation.

Gunston Polycentric Knee was the first BCR knee implant, designed by Frank Gunston in 1971. It was the first prosthesis to incorporate the natural knee biomechanics.

First-generation BCR TKA has not gained popularity due to its kinematic conflict and early failures apart from being a challenging technique and having poor survivorship. Due to implant design improvements and better instrumentation, second generation BCR TKA has seen renewed interest among orthopedists. At present only short-term data are available, but they are encouraging. Now uniformly reproducible outcomes may be offered, the prosthesis is reliable and KOOS scores are improving.

In the future robotic assistance, sensor and computer assisted balancing (SCAB) and smart instruments may help in alignment improving patient satisfaction.

Abbreviation

ACL = Anterior Cruciate Ligament.
BCR = Bicruciate-retaining.
CR = Cruciate-Retaining.
CS = Cruciate-Sacrificing/Substituting.
LFC = Lateral femoral condyle.
MLC =Medial Femoral Condyle.
PFJ = Patellofemoral Joint.
PCL = Posterior Cruciate Ligament.
PS =Posterior-Stabilized.
ROM = Range Of Motion.
SCAB = Sensor and Computer Assisted Balancing.
TCP = Total Condylar Prosthesis.
TKA = Total Knee Arthroplasty.

References

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