Dr. Patrick Meere NYU Langone Medical Center New York NY

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PATRICK A. MEERE, MD

Department of Orthopaedic Surgery

NYU Hospital for Joint Diseases

New York, NY

Hospital for Joint Diseases

NYU LANGONE MEDICAL CENTER

The Use of Sensor Technology

Allowing Implant Salvage

In Selected Cases of Revision Total Knee Arthroplasty

A TWO-CASE RETROSPECTIVE CASE SERIES

Hospital for Joint Diseases

NYU LANGONE MEDICAL CENTER

PATRICK A. MEERE, MD

Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York, NY

2

INTRODUCTION

The revision burden associated with total knee

arthroplasty (TKA) is projected to reach a staggering

13 billion dollars, annually [3, 4, 5]. Complications

reported by TKA recipients include: pain (44%

overall), sensation of instability (21% reason for

revision), and joint stiness (17% reason for revision);

problems that may be attributed to soft-tissue

imbalance [6, 7, 8]. One of the possible reasons

for the substantial prevalence of such complications

is the traditional subjectivity associated with

defining intraoperative soft-tissue balance [9].

The use of pressure sensors imbedded in tibial

trial liners is proving an ecient method for soft

tissue envelope calibration and rebalancing at the

time of TKA [2]. When used in a revision case of

a compatible implant, it may prevent the need for

the exchange of all components. It is also critical

that such methods are developed to diagnose

specific problems during revision TKA, thereby

facilitating surgical correction and implant salvage

when feasible. Therefore, the purpose of this

consecutive, two-patient case series was to test

the ecacy of using intraoperative sensing

technology to eectively guide revision surgery

in patients with chronic instability.

METHODS

A case series of two patients is presented.

The inclusion criteria for this series were:

1) satisfactory radiographic alignment,

2) suspected soft tissue imbalance in the

coronal plane.

In both cases the contingency plan consisted of

a revision total knee arthroplasty (rev TKA) with

partial or total revision of the metallic components.

The revision TKA surgery technique focused on

balance restoration through critical analysis of

pressure mapping technique of tibio-femoral

contact point loads in the functional range. Pre-

operative and post-operative functional scores

along with clinical findings were prospectively

documented.

Hospital for Joint Diseases

NYU LANGONE MEDICAL CENTER

PATRICK A. MEERE, MD

Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York, NY

3

CASE 1

PATIENT

57 year old female, 18 months after a primary total

knee arthroplasty.

CLINICAL PRESENTATION

Persistent pain, swelling and instability. Inability to

stand, walk or perform stairs without significant

pain and stiness. No improvement despite diligent

and supervised physiotherapy for over a year.

Complains of inability to fully stretch. Positive

antalgic gait with 10 degrees lack of extension

contracture, further passive flexion range up to 95

degrees. Positive chronic eusion and periarticular

swelling. No gross opening during varus or valgus

stress test. Negative sag, less than 2 mm anterior

draw in flexion. Radiographs show a satisfactory

alignment and sizing with no evidence of

malrotation (Figure 1).

INTENDED PLAN

Open arthrotomy, lysis of adhesions, probable

femoral revision with distal femoral recession to

regain flexion / extension balance.

OPERATIVE FINDINGS

At the time of surgery pressure mapping identified

the dominant instability as excessive medial

tightness in extension with an excessively high-

pressure differential: 82 lbs medially v. 23 lbs

laterally (Figure 2). The medial and lateral pressures

in flexion were tensioned appropriately (Figure 3).

Pie-crusting of the posterior medial collateral

ligamentous fibers selectively corrected the

coronal imbalance and restored complete

extension. Simple liner thickness then suced to

restore pressure balance. The resultant pressures

in supported extension were 36 lbs medially v. 32lbs

laterally and in flexion (90 degrees) 10 lbs medially

v. 13 lbs laterally (Figures 4-5). The intra-operative

PROM measured 0-123 degrees. The metallic

implant components were preserved.

FIGURE 1 FIGURE 2, extension

Hospital for Joint Diseases

NYU LANGONE MEDICAL CENTER

PATRICK A. MEERE, MD

Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York, NY

RESULTS

CASE 1

4

POST-OPERATIVE COURSE

The WOMAC scores improved from 46.2 (pre-op)

to 86.2 at 6 weeks and 88.6 at 8 months post-

operatively. The total Knee Society Score (KSS)

improved from 70 points (pre-op) to 169 points

at 8 months. The KSS pain and function separately

improved from 25 to 89 points and 45 to 80 points

respectively. The passive range of motion (PROM)

improved from 10-90 degrees (pre-op) to 0-122

and 0-127 for the same post-op intervals. The

improved range was preserved to date.

FIGURE 3, flexion

FIGURE 4, extension

FIGURE 5, flexion

Hospital for Joint Diseases

NYU LANGONE MEDICAL CENTER

PATRICK A. MEERE, MD

Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York, NY

5

CASE 2

PATIENT

77-year-old female with recent MCL injury on

10-year-old left TKA.

CLINICAL PRESENTATION

Three months of instability, persistent pain diusely

in the left knee after sustaining an accidental fall

in a department store. Prior to this the clinical

function was good and there was no radiological

evidence of wear on her posterior stabilized left

knee total knee arthroplasty performed over 10

years ago (Figure 6). Subsequent to the fall with

hypervalgus and forceful flexion, the patient

developed recurrent and persistent instability of

the left knee. Physical exam confirmed a grade II

laxity of the medial collateral ligament with dull

point tenderness at the femoral insertion point.

There was associated multidirectional instability,

recurrent eusion, synovitis and periarticular tissue

edema. The PROM was -3 to 107. The valgus stress

test at 10 degrees of flexion showed 5 mm medial

opening with a soft endpoint. On varus stress the

lateral joint opened by 3mm. The anterior drawer

test showed anterior translation by 4mm (grade 2).

The posterior draw was stopped by the post.

INTENDED PLAN

The patient was indicated for a left knee open

arthrotomy, synovectomy, and lysis of adhesions,

possible primary medial collateral ligament repair,

probable augmentation of tibial liner with soft

tissue recalibration with the use of pressure

mapping technology, possible full revision to a

Total Stabilized TKA based on medial opening

stability and ability to generate sufficient and

functional contact resting pressures.

OPERATIVE FINDINGS

The patient was found to have the anticipated

diuse synovitis secondary to chronic multidirec-

tional instability. The MCL was found to be plasti-

cally deformed by approximately 10% but with very

good residual elasticity. There was no evidence

of mechanical dissociation of the implants. The

patient was found to have significant patella baja

with arthrofibrosis of the infrapatellar ligament.

Compartmental tibial load pressure mapping

indicated insufficient load on the medial side,

reflecting the known traumatic laxity of the medial

collateral ligament in extension (Figure 7). To

increase the contact pressure to an acceptable

level, a liner thickness augmentation is required.

This however caused overstung of the lateral

compartment in extension (Figure 8). This was

then corrected by a rim coronary ligament release

(Arcuate plus 3mm of the lateral Iliotibial band

insertion up to Gerdy’s tubercle) on the lateral side

and final liner thickness augmentation by 5mm.

The coronal pressure balance was thus optimized

to less than 10 pounds dierential in extension and

in flexion (Figure 9-10).

The above pathway pertains to static loads.

FIGURE 6

Hospital for Joint Diseases

NYU LANGONE MEDICAL CENTER

PATRICK A. MEERE, MD

Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York, NY

6

RESULTS

CASE 2

Dynamic load evaluation through a varus / valgus

maneuver (at 10 degrees of flexion) can produce

pressure dierentials that reflect the tension in the

ligaments opposite to the direction of the applied

force. Varus loading with 25 N force created a

20 lbs load dierential on the medial compartment,

representing the tightness of the lateral collateral

ligament. Valgus loading with the same 25 N force

generated a 17 lbs load dierential, confirming the

sucient resiliency of the MCL. This ligamentous

stability was deemed to be very acceptable and

preferable to a full revision to a TS component.

POST-OPERATIVE COURSE

Discharged on post-operative day (POD) 2. Single

cane on POD 10. Unassisted ambulation by POD

20 with walking tolerance of 6 blocks, no assist

on stairs. Post-operative exam at 21 days reveals

minimal pain or tenderness. Medial opening on

valgus stress of less than 3mm. PROM: -3 to 112

degrees. The WOMAC score improved from 47.7

(pre-op) to 84.8 at 6 weeks.

FIGURE 7, extension FIGURE 9, extension

FIGURE 8, extension FIGURE 10, flexion

Hospital for Joint Diseases

NYU LANGONE MEDICAL CENTER

PATRICK A. MEERE, MD

Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York, NY

7

DISCUSSION

In this short case series, two patients presented

with chronic instability, pain, and eusion in an

established TKA joint. All asymmetric loading was

confirmed and corrected through the use of the

intraoperative sensor system. The initial operative

plan for both cases indicated a potential need for

exchange of metal components. However, digital

guidance provided by the sensor system obviated

the need for the surgeon to exchange any metal

components, thereby avoiding patient morbidities

and excess cost associated with revision surgery.

In Case I, the patient presented with coronal soft-

tissue imbalance, driven by excessive tension in

the posterior medial collateral ligament fibers. In

such cases, pressure mapping sensor technology

helps the surgeon to define the specific deficiency

and probable best correction. In many revision

cases, the imbalance is coronal rather than sagittal.

Thus, implant salvage may be feasible, sparing the

patient the morbidity associated with a complete

revision of all TKA components.

In Case II, the decision to convert an unstable TKA

after a serious MCL injury relied on the subjective

impression of medial joint line opening and

shearing upon valgus stress testing. The adjunct

use of sensor technology in this case allowed for

quantified evidence of the morbid imbalance and

assisted in the performance of titrated release of

the relatively tighter contralateral ligament and

capsule upon liner thickness augmentation.

Finally, the response to physiological loading of

the collateral ligaments showed symmetrical ΔP’s

(rapid pressure dierentials during the load

impulse application and recoil). This confirmed

the restoration of functional stability to the joint.

Of note was the fact that the implant salvaged in

this case was of a dierent manufacturer than that

for which the sensor was design. Nonetheless,

undersizing by one size allowed for a good fit

and translational stability for compression load

testing purposes.

This small case series provides promising results

for the ecacy of using intraoperative sensor

technology during revision cases requiring the

correction of instability. Further case studies and

longer follow-up will need to be obtained to

understand long-term outcomes.

Hospital for Joint Diseases

NYU LANGONE MEDICAL CENTER

PATRICK A. MEERE, MD

Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York, NY

8

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