MEMORANDUM

Reclassification Petition: Clinical Consult Review

 

To:      Orthopaedic Advisory Panel

From:  Orthopaedic Devices Branch, Division of General, Restorative and Neurological Devices;

Date:   April 26, 2004           

Subject:  Reclassification Petition: Mobile Bearing Knee Prostheses       

 ____________________________________________________________________________________

 

Summary: 

This reclassification petition presents data to support the petition for reclassification of all mobile bearing unicondylar, bicondylar condylar and tricompartmental knees from Class III to Class II.  This includes clinical data from IDE and outcome studies, peer reviewed journal articles and meta- analysis comparison analyses between fixed and mobile bearing systems related to adverse events and effectiveness outcomes. The sponsor believes that this information provides strong evidence of the safety and efficacy of mobile bearing knees, and that the risks associated with these devices are now adequately defined. Before a decision can be made whether reclassification from Class III (Premarket Approval Application) to Class II (Special Controls) is justified, further information and clarification is needed.

 

Background:

Reclassification is requested from Class III to Class II.  

There are numerous mobile bearing knee designs on the market worldwide

designed to increase contact area and /or to reduce implant-to-bone interface stresses by allowing mobility of the polyethylene bearing on the tibial plate in order to potentially reduce long-term wear.

 

Regulatory History of the Device Class

FDA issued a Proposed Rule classifying 77 orthopedic devices on July 2, 1982 (47 FR 29052), and the Final Rule on September 4, 1987 (52 FR 33686). The Final Rule established twelve separate categories of implantable knee prostheses (21 CFR 888.3480 through 888.3590). Each of the twelve types of knee prostheses described were assigned to either Class II or Class III depending on system attributes such as fixation method, level of constraint, and degree of resurfacing (e.g., patellofemorotibial versus femoral).

 

None of the categories, however, describes the type of total knee replacement system that has come to be known as the mobile bearing knee.

 

The question of reclassification of mobile bearing knees from Class III (Premarket Approval) to Class II (Special Controls) was considered by an FDA Advisory Committee on January 13, 1998. At that time, the panel's recommendation was to retain the Class III designation for all tricompartmental and unicompartmental mobile bearing knees with the exception of recommending reclassification of tricompartmental mobile bearing knees

that are cemented and have a rotating/translating base. The panel also recommended post-market surveillance for those mobile bearing knees reclassified to Class II. The FDA subsequently chose to recommend submission of a new reclassification petition for the entire class of mobile bearing knees, rather than reclassify specific subcategories.

 

Device History

The first mobile bearing knee designs were introduced in the late 1970's. The Oxford Unicondylar knee (Biomet, Inc., Warsaw, IN) was the first to utilize a mobile bearing to reduce contact stress while also reducing implant-to-bone interface stress. Since those early implants, several generations of mobile bearing knees have followed, and today there are nearly 50 unicondylar and bi-condylar implant designs with either platform-style or meniscal bearing design of the polyethylene articulating surface on the international market. There are numerous variations in the directional mobility of the polyethylene, type of constraint of the polyethylene, and treatment of the PCL.  The first mobile bearing knee to be approved by the FDA for sale in the U.S. was the Low Contact Stress (LCS) Rotating Platform Knee (J&J DePuy, Warsaw, IN). PMA approval for this knee was received in 1985, and since then five other mobile bearing knees have been approved in the U.S.

 

Reclassification of several types of knees from Class III to Class II was considered by an FDA Advisory Panel on January 13, 1998 ("Petition for Reclassification, Patello-Femoral-Tibial Metal/Polymer/Metal/Polymer/Metal Biologically Fixed Prosthesis, submitted by the Orthopedic Surgical Manufacturers Association, July 25, 1997). Mobile bearing knees were included in that petition. At that time, the Panel believed there was insufficient evidence to provide reasonable assurance of safety and efficacy for the entire class of mobile bearing knees. They recommended reclassification only of tricompartmental mobile bearing knees that are cemented and have a rotating/translating base. However, they recommended the retention of Class III designation for all other tricompartmental and unicompartmental mobile bearing knees.

 

 At this time 6 devices are available in the US:

• Low Contact Stress (LCS) Rotating Platform

• P.F.C. Sigma Rotating Platform

• P.F.C. Sigma Stabilized Rotating Platform

• Low Contact Stress (LCS) Meniscal Bearing

• Low Contact Stress (LCS) Unicompartmental Knee

• Oxford Meniscal Unicompartmental Knee

 

Intended Use:

The intended use differs depending on the subtype.

 

The mobile bearing total knee, fixed with or without bone cement is indicated for:

• Patients with knee pain and disability due to rheumatoid arthritis, osteoarthritis, traumatic arthritis, polyarthritis, collagen disorders and/or avascular necrosis of the femoral condyle

• Post-traumatic loss of joint configuration (particularly when there is patellofemoral erosion, dysfunction, or prior patellectomy)

• Moderate valgus, varus, or flexion deformities

• The salvage of previously failed surgical attempts if the knee can be satisfactorily balanced and stabilized at the time of surgery

 

The mobile bearing unicompartmental Knee, fixed with or without bone cement is indicated for:

• Patients with knee pain and disability due to osteoarthritis or traumatic arthritis

• Previous tibial condyle or plateau fractures with loss of anatomy or function

• Varus or valgus deformities

• Use with an intact Anterior Cruciate Ligament (ACL)

• Revision of previous unicompartmental arthroplasty procedures

 

Device Description/Principle of Operation:

This class of devices includes two subtypes: bicondylar knee joint patellofemorotibial metal/polymer mobile bearing cemented or porous coated uncemented prosthesis" and

Unicondylar knee joint femorotibial (uni-compartmental) metal/polymer mobile bearing

cemented or porous coated uncemented prosthesis"

 

The defining feature of a mobile bearing knee is the presence of a moving polyethylene bearing that articulates with both the femoral condyle and the tibial tray.  It is theorized that mobile bearing knee designs potentially reduce polyethylene wear by their highly conforming surface that disperses contact forces over a large area. The mobility in the polyethylene bearing, which reduces implant-to-bone interface stresses, may prevent implant loosening, which has been attributed to high interface stresses in highly conforming fixed bearing knee designs.

 

Mobile bearing knees are available in PCL-retaining, PCL-sacrificing and PCL stabilizing designs. In general, knees with only rotating mobility utilize a PCL sacrificing or PCL-stabilizing design, while multidirectional platform knees generally are PCL-retaining.

 

These devices can be further categorized by the type of bearing surface (Platform, bicondylar meniscal bearing, and unicondylar meniscal bearing), type of constraint and type of fixation (cemented , porous coated uncemented).

Bearing Surface included:

• Platform: a single polyethylene bearing that rotates in the transverse plane, with or without A/P motion (rotating only or multidirectional).
• Meniscal Bearing: separate medial and lateral polyethylene bearings that slide

independently in arced tracks that run anteriorly and posteriorly in the fixed, metal

tibial component

• Unicondylar Meniscal Bearing: an implant in which only the medial or lateral

compartment of the knee is replaced. The polyethylene may run in a track as

described above, or may move freely, held in place only by its reciprocal shape and

the tension of the surrounding ligaments.

 

Past experience and literature have shown that the use of cement has improved results and higher failure rates are associated with non cemented use.  In light of this information, it should be considered whether we can  include uncemented (porous coated) devices with cemented devices (i.e. do we have enough data to feel comfortable that special controls will predict safe use).

 

Type of Constraint (prevention of bearing dislocation)

Unconstrained designs have very low constraint forces over the entire range of normal (physiologic) displacements.

Semi-constrained designs have near physiologic constraint that rises over the range of normal displacements.

Constrained designs have constraint forces that exceed physiologic levels and rise sharply over the range of displacements

• Cone-in-cone Design: incorporates a tapered projection of the polyethylene insert that  inserts into a reciprocal concavity in the tibial tray
• Tibial Tray Post: A post extending from the superior surface of the tibial tray fits into a recess on the polyethylene insert
• Longitudinal Curved Sliding Tracks: Movement of the platform or meniscus is limited by a track formed in the upper surface of the tibial tray
• Stops: elevated rim of the tibial tray that limits excessive A/P translation or rotation
• Unconstrained Bearing: designs that lack a mechanical limit to movement, but instead rely on the conformity of the polyethylene mobile bearing to the femoral condyle and  the tension of the soft tissues

 

Characterization of the Directional Mobility of the Bearing Surfaces:

 Nine knee designs were tested in a dynamic testing system in which compressive load was applied as the knee was rotated or moved in the A/P or M/L direction. Torque or shear forces were measured and plotted against displacement, thus characterizing the ability of the knee design to constrain displacement during gait. The nine designs were then characterized as "unconstrained", "semiconstrained", and "constrained". Of the nine designs evaluated, all demonstrated unconstrained motion in the rotational direction and all the mobile bearing designs

demonstrated unconstrained mobility within a total of 15 degrees internal/external rotation. Relative to M/L mobility, the designs tested were evenly divided between semi-constrained and constrained mobility. Constrained and semiconstrained M/L mobility is characteristic of both mobile and fixed bearing knee designs, and does not adversely affect clinical performance.  Analysis of A/P mobility revealed a wide range of constraint, with unconstrained designs prevailing. In order to achieve joint stability with the lower level of constraint, competent soft tissue, including balanced collaterals and/or the PCL are necessary.

 

• Congruence:

Fully congruent mobile bearing knees are those that have a high degree of conformity between the femoral condyle and the polyethylene bearing surface, over a wide range of flexion (approximately 120 degrees). accomplished by providing a constant sagittal femoral radius, limited by posterior impingement of the tibial component. A fully congruent prostheses has a large contact area between the femoral condyle and the bearing surface, disperses contact forces, thereby theoretically resulting in reduced polyethylene wear.

 

Gait congruent or partially congruent mobile bearing knees have large contact areas in the first 20 degrees of flexion. The contact area decreases with flexion due to a decreasing sagittal radius. These prostheses maximize contact areas in the low end of the flexion range, while decreasing the sagittal radius to improve flexion range.

The term "functionally congruent” is used to mean that a device has a single femoral radius for up to 75 degrees of flexion.

 

Summary of Unpublished IDE data

	BIOMET TRAC PS Rotating Platform	OXFORD UNICOND Phase II Meniscal Bearing	S&N GEN II Rotating P, Multidirectional P	PROFIX RP MP	STRYKER HO SCORPIO PS RP	ZIMMER MOBILE BEARING KNEES MP	ZIMMER NEXTGEN LPS FLEX RP
Type	P, MC, Rand	P,MC	P,MC,R	P,MC,R	P,MC,R	P,MC, NR	P,MC,R
#enrolled	130 (all cemented)	125 knees(all cemented) 107 pts	109/119 cemented 94/104knees uncemented 15 hybrid	59 50 cement 9 hybrid	62	179	61cemented
1 yr	103	104/125	106 pt		41 pts @ 7 weeks; (6 mo 12pts)	145	81/123(65%)
2 yr (%)	89		3			51%
3 yr	333
4 yr f/u	6
Mean F/U	2.2 yr	4.8	1 yr	9 mo	3 mo	1.5 yr
Age Mean	65 (40-87)	62 (34-85)	64-65	64	63	65	62
KSS/HSS Good-Excellent	KSS 1 yr     77.4% 2 yr     83.7% 3 yr     85% 4 yr     100%	HSS 95.2 95 14% required walking support	89= ave KSS	KSS 92%	75% 6mo	KSS 89.7 89.3	KSS 84%
Revisions	7	16 (7?) survivor ship 94% @ 6 yrs 88% cumulative @ 6yrs	2(1yr)	0	0	3	0
Reasons	-Poly  (multiple) dislocation(11) -Insert subluxation (1) -Pt dislike	-Poly dislocation(4%) -Deep infection -Lat  compartmt degeneration -Aseptic loosening :femur and tibia -poly wear -patellar complication -RA onset -Auto Accident -Patella Dislocat -improper alignment -meniscal cyst 1 pt death	-deep infection(1yr) -poly subluxation(3 mo) -Fracture -Fibroarthrosis -RSD