total knee replacement

Conformity

Definition

Geometric measure of the closeness of the fit of the knee articulation

Concepts

The ideal TKA maximises articular conformity while minimising axial constraint

Highly conforming / Fully congruent

- constant sagittal femoral radius

- large contact areas

- theoretical limitation of flexion to 120^o due to posterior impingement of the tibial component

Philosophy

Need the least amount of constraint necessary to obtain sufficient stability

Increasing constraint

Advantages

- increase stability

Disadvantage

- increase stress at implant host interface

3 Types

1.  Unconstrained

A.  Posterior cruciate retaining

Options

1. Always resurface

2. Never resurface

3. Selectively resurface

Decision Making

Controversial

- literature divided on issue

Historically

- poor outcomes due to poor implant design

- now improved designs

- non resurfacing also improved due to better design and improved techniques in regard to tracking and rotation

Intraoperative Fractures

1.  Shaft fracture from IM rod

2.  Posterior condylar fracture

Management

- assess stability

- ORIF if needed

Incidence

10%

Requirements

70^o  swing phase

80^o climb up stairs

90^o climb down stairs + sit down in chair

100^o low chair

NHx

Stiffness usually subsides at 6-8/52

- generally improves out to 3/12

- slow improvement for up to next 9/12

Arterial injury

Incidence

0.05%

Types

1. Thrombosis

- most common scenario

- tourniquet with atherosclerosis

- indirect damage to vessel

- femoral or popliteal

2.  Embolisation

- atherosclerosis again

- blue toes syndrome post operatively

3.  Vessel transection

Incidence

10 -15%

Include

- marginal necrosis

- wound slough

- sinus tract formation

- dehiscence

- haematoma

- oozing knee wound

Blood supply

Anterior knee has no muscles to supply vessels directly 

Incidence

Knee > Hip

- superficial position

- limited cover of well vascularised muscle 

- watershed area of skin blood supply anterior to the skin incision 

- much increased in fully constrained prosthesis 

Ideal < 1%

Risk Factors

Increased with

- revision

- prior infection

- RA / Psoriatic arthropathy

- DM