Arthroplasty Flashcards
Vancouver Classification for periprosthetic THA and Tx?
A: troch fx, if less than 2 cm displacement can tx nonop
B1: Stable stem, ORIF with locking plate and cables
B2: loose stem, long porous coated or MFTS
B3: poor bone stock and loose, Femoral revision with proximal femoral allograft or PFR
C: Fx well below prosthesis, ORIF with plate
Risk factor for intraop femoral fx during THA?
Female*** Anterolateral approach**** (vs posterolateral) MIS surgery Cementless stem*** Revision*** Metabolic bone disease***
Intraop calcar fx treated with cerclage does NOT increase risk of component subsidence or failure in long term f/u
Polyethylene sterilization
1-Radiation, oxidation vs cross linking, removal of free radicals
2 - how to solve these issues?
1- Gamma radiation is MOST COMMON form of PE sterilization –> get oxidized PE which wears poorly and causes osteolysis***
Oxidation vs cross linking
O2 rich environment –> PE becomes oxidized –> early failure 2/2 delimitation, pitting, fatigue strength/crakcing
O2 depleted environment: PE becomes CROSS LINKED –> better resistance to adhesive and abrasive wear*
Decreased Mechanical properties –> decrease4d ductility and fatigue resistance! –> higher risk of catastrophic failure under high loads
Must package in argon or nitrogen or in vacuum*
Free radical removal:
Thermal stabilization/remelting –>? removes free radicals made during radiation sterilization for cross linking –> most effective as it occurs above PE melting point* –> changes PE from partial crystalline state to amorphous state –> this REDUCES mechanical properties*
Annealing: MAINTAINS MECHANICAL PROPERTIES –> less effective at removing free radicals! –> susceptible to oxidation ***
2 - Solution: Irraiate PE in INERT GAS (nitrogen or argon) or in vacuum to minimize oxidation***
PE manufacturing fabrication methods
Causes of failure?
Solution?
Manufacturing Methods
1-Ram bar extrusion and machining
UHMWPE powder into heated chamber, ram pushes into heated cylinder barrel –> cylindrical rod –> 10 ft length –> implants from bar stock –> get VARIATIONS in PE quality w/in bar***
2- Calcium stearate additive –> leads to fusion defects in PE
3- Sheet compression molding: UHMWPE powder into 4’ x 8’ rectangular container to make sheets up to 8” thick –> implants from molded sheets
4 - direct compression molding/net shape
UHMWPE poser into molded shape of final component –> heated –> BEST PE FABRICATION PROCESS*** –> lower wear rates but slow and expensive
Failure
Machining shear forces causes subsurface region stretching of PE chains
PE more susceptible to XRT in this region –> more oxidation –> delimitation and fatigue cracking –> Classic white band of oxidation 1-2 mm below articular surface***
Perfect storm for catastrophic wear:
Metal backed tibia w/ bone conserving tibial cut = thin PE
Flat bearing design –> low contact area with HIGH contact load
PCL retention w/ flat PE –> high sliding wear
Ram bar PE w/ calcium stearate additive –> fusion defects in PE
Gamma rad in air –> weakened mech properties of PE (oxidation)
Machine PE surface –> cutting tool stretch effect on PE
Solution
Use DIRECT COMPRESSION MOLDING of PE
Less fatigue crack formation and propagation vs ram bar extrusion*
Avoid machining of articular surface
What process changes PE from partial crystalline to amorphous state?
What process most increases wear resistance for UHMWPE?
Does annealing or remelting decrease free radicals more?
Crystalline to amorphous - Remelting***
Increases wear-resistance: Radiation –> cross linking***
Remelting reduces free radicals more***
What wear rate is ass’d with osteolysis and component loosening?
What component factor most determines wear rate for THA?
Linear wear rate > 0.1 mm/year***
Most important factor = head size***
V = 3.14r^2w (V = volumetric wear, r = head radius, w = linear head wear)
HOWEVER –> femoral head sizes between 22 and 46 do NOT influence wear rates appreciably for UHMWPE**
What factors cause greater MoM serum ion levels?
Cup abduction >55 degrees***
Smaller component sizes***
What factors do macrophages release to cause osteolysis in total joint?
Osteolytic factors/cytokines
TNA-alpha*
IL-1*
IL-6***
Increase in TNF-alpha INCREASES RANK*
increase of VEGF with UHMWPE enhances RANK and RANKL activation –> RNAKL mediated bone resorption*
How to measure bone turnover on labs?
N-telopeptide urine levels***
What size of PE particles are the most reactive?
<1 micron* –> most reactive to macrophage induced osteolysis*
What is anakinra?
What is tocilizumab?
Receptor antagonist of IL-1**
IL-1 = PRO inflammatory cytokine –> causes osteolysis***
tocilizumab = monoclonal Ab against IL-6 (another pro inflammatory cytokine that causes OSTEOLYSIS)
How much wear per year puts a prosthetic joint at risk for failure?
0.1 mm/year***
What is a normal Tonnis angle?
Measures angle of the weight bearing surface
Angle between horizontal line from medial and lateral edges of sourcil***
Normal about 10 degrees***
Lateral center edge angle?
Measures femoral head lateralization on AP pelvis
Angle formed by intersection of vertical through center of head and line extending from center of head to lateral sourcil***
Normal 25-45; <20 DIAGNOSTIC for DDH
Anterior center edge angle?
Measures ANTERIOR DYSPLASIA on FALSE PROFILE
Angle between vertical line through center of head and line going from center of head to anterior sourcil***-
normal: 25-50 degrees; <20 diagnostic for DDH
What does alpha angle check on frog leg lateral?
checking for FAI secondary to femoral head/neck offset deformity***
Anything over 42 degrees = FAI***
Cell count for infection in TKA?
Acute infection numbers?
For hips?
MoM hips?
Cell count (long standing joint) for TKA –> 1100 cells and 64% PMN***
Acute infection: 27,800 cells in first 6 weeks after TKA** (basically 30,000)
Hips: WBC >3000 and PMN >80%***
MoM hips: 4350+ cells and 85% PMN***
What does alpha anti-defensing test for?
Presence of an INTRAARTICULAR, antimicrobial peptide**
How long can an infection be considered a surgical site infection (SSI)?
one year***
What inflammatory marker has the highest correlation for PJI?
IL-6 ***
What can decrease joint reactive forces? (Acetabulum, femur, gait)
What increases forces?
Decrease joint reactive forces by shifting center of rotation medially* –> do this by:
Acetabulum: Move acetabular component medially, inferiorly and anterior*
Femur: Increase offset***
Long sem prosthesis
Lateralization of GT
Gait: shifting body weight over affected hip –> Trendelnburg gait*
Cane in contra hand* –> reduces abductor muscle pull and decreases moment arm between center of gravity and femoral head –> cane creates additional force that keeps pelvis level during unilateral stance***
Increase joint reactive forces: Valgus neck-shaft angulation –> BUT decreases shear***
How to determine joint reactive forces (equation)?
Joint reactive force = Abductor tension + 5/6(body weight) ***
Abductor tension (distance from center of rotation to greater Troch) - (5/6BW x distance from center of head to center of pubis)***
What happens in Trendelenburg gait?
The pelvis on swing side drops, causing increased adduction of the affected hip during the stance phase***
Lurch of trunk towards affected side (during stance)
Normal Q angle in extension of males? females?
Males - 13 degrees***
Females 18 degrees***
What is “screw home” mechanism of knee mechanics?
Tibia EXTERNALLY ROTATES in last 15 degrees of extension***
Due to medial tibial plateau being longer than lateral plateau***
Relevance: “locks” knee in extension to decrease work performed by quad during stance
Normal tibia-femoral joint kinematics with flexion/extension
tibia EXernally rotates on femur as the knee EXtends*** (screw home mechanism)
Tibia internally rotates during knee flexion***
Medial femoral condyle does not move much from 0-120 degrees, but lateral femoral condyle moves posteriorly***
Then both move posteriorly from 120 degrees of flexion on***
What is “paradoxical motion” used to describe knee kinematics?
Used to describe ACL deficient CR TKA***
Femur usually has “rollback” –> posterior movement of tibiofemoral contact point with knee motion from extension to flexion
In “paradoxical rollback” –> ACL deficient CR knee can’t create normal femoral rollback w/ knee flexion*** –> get ANTERIOR contact movement (happens more on medial side than lateral in kinematic trial)
Risk of femoral head collapse based on imaging - how to predict?
Based on modified Kerboul necrotic angle –> add arc of femoral head necrosis on mid-sagittal and mid-coronal MR images***
Low risk: <190 combined degrees
Moderate: 190-240
High risk: >240 degrees***
