How Accurate Is Patellar Height Measurement Using

DigitalImaging?

BA Rogers1, NJ Little2

1St George’s Hospital, London2Epsom Hospital, Surrey

IMECHE, London, 2009

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 knee

arthroplasty

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