Intra- & Inter- Observer Variation in the Measurement of Patellar Height

After Total Knee Replacement Using Digital Radiography

BA Rogers1, NJ Little2

1St Peter’s Hospital, Chertsey, England2Waikato Hospital, Hamilton, New Zealand

SICOT, Hong Kong, August 2008

Introduction

• Patellar height crucial for extensor mechanism

• Patella alta (high) & baja (low) – associated with pathology

• Difficult to define femoral condylar groove radiologically

• Several ratios relate patella to proximal tibia

Patellar Height Measurement

Insall-Salvati Mod. Insall-Salvati Blackburne-Peel Caton-Deschamp

Introduction

• Figgie et al. (J. Bone and Joint Surgery. 1986;68-A:1035-40)

– N=101– Associated poor function & Ant knee pain with

patella baja

• Koshino et al. (J. Arthroplasty.1990;5:323-327.)

– N=94, using Insall-Salvati– 65% incidence of patella baja post TKA

(Kinematic)

Introduction

• Grelsamer et al. (J. Arthroplasty. 2002;17:66-9.)

– Accounting for poly insert in TKA– Proximal transfer of joint line– aka Pseudo-baja– Concluded

“ true patellar baja cannot be measured post-TKA using either Blackburne-Peel or Caton-Deschamp”

Aim

This study compares reliability of all fourpatellar height ratios pre- and post- total kneearthroplasty

1. Using film and digital imaging2. Measuring inter- and intra- observer

correlation

Methods

• 48 patients • TKA for OA or RA• Kinemax (Howmedica) or PFC (DePuy)• Revision or previous HTO - excluded• 2 examiners

– Identical conditions

– Sequential measurement

Methods

• Digital & Plain imaging• Lateral knee XR, 20° + flexion• IS, mod IS, BP & CD ratios measured• Pre- & Post- TKA• Repeated at 3 months• 3072 measurements per examiner• Statistical analysis:

intra-class correlation coefficient (ICC)

  

48 Knees : Osteoarthritis or Rheumatoid Arthritis

2 Independent Observers

1. Caton – Deschamps 2. Blackburn – Peel 3. Insall – Salvati 4. Mod Insall - Salvati

Pre-Op Patellar Height Measurement Digital Imaging Plain XR films

1. Caton – Deschamps 2. Blackburn – Peel 3. Insall – Salvati 4. Mod Insall - Salvati

Post -

-Op Patellar Height Measurement Digital Imaging Plain XR films

PFC / Kinemax TKA

Repeated at 3 months

Repeated at 3 months

Results

• Digital imaging superior

• Blackburn-Peel & Caton- Deschamps higher ICC, especially with digital

• Greater improvement with digital seen with Insall-Salvati ratio

• Intra- & Inter observer results similar

Results

0

0.05

0.1

0.15

0.2

0.25

IS mod IS BP CD

Cha

nge

in IC

C

Pre-TKR

Post – TKR

Improvement in intra class correlation for inter-observer error using digital radiography both pre- and post- TKR

Discussion

• Magnification & contrast adjustment

• More accurate identification of landmarks

• Especially tibial tuberosity (IS & mIS)

Multiple Anterior Tibial Notches

Convex Anterior Tibial Surface

Articular surface of patella not visible

Assessment of films showing patellar tendon insertion separate from anterior tibial notch or flare

 

N=15 Pre-Op/mm Post-Op/mm

Range of patella tendon insertion *

-3 to +10 -4 to +13

+ve = insertion proximal to notch, -ve = distal

Example of patella tendon inserting proximal to anterior tibial notch

Discussion

• Poly insert makes BP & CD methods more reliable post TKR

• No improvement for IS & mIS methods post TKR since directly measure patella tendon

Conclusion1. The theoretical advantages of the Insall-Salvati and modified

Insall-Salvati ratios, measuring true patellar height following

TKA, need to be balanced against their inferior correlation

in comparison to other ratios

2. Digital imaging affords a non-uniform improvement with

the Caton-Deschamps and Blackburn ratios, when measured using

digital radiography following TKR, exhibiting a very high correlation.

Thank you