A NEW PROSTHESIS INTERFACE GEOMETRY DESIGNED TO PROVIDE IMMEDIATE STABILITY AND A GRADUATED STRESS DISTRIBUTION TO PROMOTE INGROWTH FIXATION (INSTACONE KNEE)

Participants: B. Hallstrom, S.A. Goldstein, L.S. Matthews, J.M. Hall

Keywords: knee prosthesis, ingrowth fixation, total knee arthroplasty

Introduction

A new prosthesis interface surface (Instacone Knee System) was designed to use an array of porous coated cones for immediate surgical component stabilization and to encourage strong durable biologic fixation. A conventional marketed human tibial component (Genesis Knee System, Smith-Nephew Richards Medical Company) was modified by the addition of the experimental surface using all manufacturing standards applicable to similarly marketed devices.

Preliminary implantation in human cadaver tibias demonstrated excellent immediate stability, and in vivo implantation in dogs demonstrated rapid secure ingrowth and advantageous reorientation of tibial trabecular bone toward the stabilizing cones.

A pilot human clinical study was then conducted to determine whether the new knee prosthesis fixation surface design (Instacone Knee) provides a better and a more durable bond between the prosthesis and the patient’s bone, and whether a prosthetic part with this new surface stimulates bony ingrowth and positive trabecular remodeling, resulting in more reliable pain relief and better function for a longer time than the presently marketed standard cemented design (Genesis Knee).

Materials and Methods

Approval to test the system in humans was obtained from the Food and Drug Administration (FDA) and the University of Michigan Institutional Review Board (IRB) prior to initiation of the study..

Patients presenting to the University of Michigan Orthopaedic Service who were appropriate candidates for uncemented total knee replacement were invited to participate. Patients with osteo or rheumatoid arthritis were included in the study. Patients with neuromuscular disease, Charcot joints or gross metaphyseal bone loss were excluded. Other exclusions included host conditions unlikely to allow for bony ingrowth (bony loss, poor regional blood supply, poor bone quality), active infection, patients unable or unwilling to comply with postoperative restrictions, patients involved in other investigational studies, patients with acquired immune deficiency syndrome, and patients whose age, weight or activity level were not compatible with satisfactory long term results.

Clinical and radiographic assessment were conducted preoperatively and at 6 weeks, 3 months, 6 months, 12 months and 24 months postoperatively. The clinical assessment was completed on a standardized form following the Knee Society clinical rating system.¹ The radiographic evaluation consisted of an AP and lateral knee view with a patellar skyline, or Merchant's view.

Descriptive statistics were run on the full sample of 19 knees. Statistical comparison of clinical measures at baseline (pre-op) and 12 month follow-up were conducted using a nonparametric repeated measure model (Wilcoxan Signed Rank test). To meet the assumption of independent observations, one set of data from patients with bilateral implants was randomly selected to be included in the analysis, resulting in a sample size of 16.

Results

Sixteen patients (12 men and 4 women), with a mean age of 61 ± 6 years (range 47-70 years) had Instacone knees implanted by a single surgeon between October 1994 and May 1997. Three patients had bilateral Instacone knees implanted during separate surgeries at least one year apart, resulting in a total of 19 implants. Twelve month follow-up data were obtained on all 19 knees.

Pain was significantly decreased between pre-op and 12 months post op (p=0.001) with mean pain scores of 5.7 at pre-op and 2.2 at 12 months. (Pain Scores: 1=None, 2=Start-up pain, 3=Stairs only, 4=Walking & stairs, 5=Occasional, 6=Continuous, 7=Severe.) Walking was also significantly improved at 12 months compared to pre-op (p=0.002), with mean scores of 3.2 pre-op and 1.3 at 12 months. (Walking Scores: 1=Unlimited, 2= >10 blocks, 3=5-10 blocks, 4= <5 blocks, 5=Housebound, 6=Unable.) Similarly, stair climbing was significantly improved at 12 months compared to pre-op (p=0.002), with mean scores of 2.8 pre-op and 1.35 at 12 months. (Stairs Scores: 1=Normal up & down, 2=Normal up, down w/ rail, 3=Up & down w/ rail, 4=Up w/ rail, unable down, 5=Unable.) Range of motion was not significantly different at 12 month follow-up (100 degrees) compared to pre-op (103 degrees).

Progress

Clinical and radiographic follow-up data will continue to be collected. Currently, two year follow-up data have been obtained on 18 of the 19 knees with Instacone implants, three year follow-up data have been obtained on 8 knees, and four year follow-up data have been obtained on 3 knees.

References

Insall JN, et al: Rationale of the Knee Society Clinical Rating System. Clin Orthop 248: 13-14, 1989.