8) Casting Techniques and Orthotic Fabrication Flashcards
Impression (negative) casting technique
- Neutral suspension technique
- Prone technique
- Vacuum technique
- Semi - weight bearing technique
Neutral suspension technique materials
- Plaster or STS casting sock
Prone technique materials
- Plaster or STS casting sock
Vacuum technique materials
- Plaster or STS casting sock
Semi - weight bearing technique materials
- Plaster or foam
Why impression casting?
- Necessary for the fabrication of an orthotic device
Orthoses
- Prescription medical devices which alter lower extremity alignment and function
Basic types of orthoses
- Functional
- Accommodative
Functional orthoses support/balance
- Existing forefoot deformity
- Eliminates the need for rearfoot compensation
Functional orthoses promote/limit
- Subtalar joint motion
- Stabilizes (“locks”) the midtarsal joint
Functional orthoses design
- Rearfoot posted
- Rigid / semi-rigid materials (traditionally)
- Derived from a NWB neutral cast
Functional orthoses indications
- Forefoot deformity
- Rearfoot deformity
- Postural instability
- Abnormal transverse plane leg rotation
- Subtalar joint hypermobility
- Limitation of subtalar joint motion
Accommodative orthoses
- Redistribution of plantar pressure
- Provides arch support
- Promotes / limits subtalar joint motion to some degree
Accommodative orthoses design
- Flexible materials generally utilized
- Derived form a NWB or semi-weight bearing casting technique
Accommodative orthoses indications
- Neuropathy
- Painful plantar lesions
- Increased shock absorption
- Subtalar joint hypermobility
- Postural instability
Neutral suspension casting technique
- Most commonly utilized method
- Most technically difficult method
- Excellent visualization of the subtalar joint neutral position
Neutral suspension technique limb and practitioner positioning
- Patient supine with leg extended
- Knee maintained in the frontal plane
- Practitioner may be seated or standing
- Thumb placed in sulcus of 4th and 5th digits
- Subtalar joint palpated and neutral position identified
- Forefoot loaded (midtarsal joint locked)
Neutral suspension casting STJ positioning
- Foot must be placed in “neutral” position
- STJ in neutral ~1/3 eversion:2/3 inversion
- Look at concavity/convexity over sinus tarsi
- Palpate dells of tarsal canal
Neutral suspension casting MTJ, OMTJ, LMTJ positioning
- MTJ locked
- OMTJ maximally pronated (abduction, dorsiflexion)
- LMTJ maximally pronated (supinatus)
Neutral suspension casting patient positioning
- Relaxed, comfortable and encouraged not to help
- Casted leg internally rotated
- Knees flexed
- Foot in neutral position
Neutral suspension casting plaster application
- Two piece method
- Tuck excess in toe sulcus medially
- Do not allow arch to bowstring
- Capture curvature below fibular malleolus
Prone technique
- Very common
- Less difficult than neutral suspension
- Excellent visualization of STJ neutral
- Excellent when the patient is large
Evaluation of the negative: lateral border
- Should be straight
Exceptions:
- Metatarsus adductus
- Large muscle belly
Evaluation of the negative: 5th toe position
- Indicates position of the OMTJ
Evaluation of the negative: 1st ray
- Contour of dell
- Skin lines along the declination of the first metatarsal
- No transverse lines proximal to head
- Indicates position of the LMTJ
Evaluation of the negative: contour of the heel pad
- Trisect the heel
- Middle 1/3 should be flat
- Medial 2/3 curved (supination of rearfoot)
- Evaluates the STJ position
Semi-weight bearing technique
- Commonly utilized
- Technically easy
- Difficult to maintain STJ neutral
- Indicated primarily when accommodation is desired
Semi-weight bearing technique is indicated primarily when
- Accommodation is desired
- Biofoam
Vacuum technique
- Uncommon method
- Minimal difficulty, but special equipment required
- Excellent visualization of STJ neutral
- Excellent when tight or specialized shoes are to be worn (skates, ski boots, high heels, etc.)
Considerations when pouring the positive
Calcaneal bisection
- Dell below lateral malleolus)
Data from bioeval
- What is NCSP?
- Does foot pronate to perpendicular?
- Is it maximally pronated at ?
Pouring the positive
- Separating medium
- Wedge forefoot to place rearfoot in desired position (can invert up to 4°)
- Apply dye to plaster
- May reinforce with tongue depressor
Pouring the negative: vertical
- When STJ motion is adequate and the calcaneus can evert beyond perpendicular
Pouring the negative: inverted
- When STJ motion is limited and the calcaneus cannot evert to perpendicular
Pouring the negative: everted
- When STJ is in a fixed everted position or cannot invert to perpendicular
Preparing for balancing
- Negative is removed
- “Menesci” rasped away
- “Fabricot” is used to smooth the surface
- Dye in positive prevents removal of excessive plaster
Concept of balancing
- Provide MTJ control
- Custom made walking surface
- Eliminate compensation which results in abnormal foot mechanics
Why intrinsically balance?
- To support the forefoot deformity and prevent midstance STJ compensation
- Brings the “ground up to the foot”
- Enables better fit in shoe gear
Extrinsic balancing
- No balancing of positive cast
- Forefoot balance platform added to orthotic plate
Extrinsic vs. intrinsic balancing
- Best shoe fit = intrinsic
- Fewest problems = intrinsic
- Best support of forefoot deformity = extrinsic
When rigid forefoot valgus or varus exists, use
- Extrinsic post
When the forefoot deformity is > 5 degrees, use
- A combination of balancing techniques
Steps in preparing the positive
- Forefoot balance platform (supports the forefoot deformity)
- Medial expansion
(allows for expansion with normal midstance pronation) - Lateral expansion
(fat pad displacement)
Forefoot balance platform purpose
- To support the forefoot deformity
- Identify forefoot contact points
- Reference levels for balancing
- Size of platform must be proportional to foot
- Define medial edge of orthotic
1st and 5th metatarsal heads on forefoot balance platform
- 1st met head: 12 to 15 mm squares
- 5th met head: 3/4 size of medial platform
Forefoot balance platform outcomes
- 1 to 5 balancing (usually)
- 2 to 5 balancing: metatarsus primus elevatus, plantarflexed 2nd ray deformity
- 1 to 4 balancing: 5 th metatarsal elevatus, plantarflexed 4 th metatarsal
Identifying the distal heel cup line of the forefoot balance platform
- 1.5 to 2 cm proximal to calcaneal cuboid joint
- Locates lateral border of medial expansion and medial border of orthotic
- Wider orthotic = better control
A “balance” nail is used to
- Place forefoot contact parallel to ground
- Forefoot valgus: balance nail under 5th met head
- Forefoot varus: balance nail under 1st metatarsal
Tools for platform construction
- Plaster
- Spatula
- Waxed paper
Construction of the platform
- Forefoot of positive in plaster
- When semi-set, trim proximal aspect using reference points
- On supporting surface, superior aspect of cast in balanced position
- Trim to medial and lateral border
- Sand lightly to expose balance nail
Medial expansion plaster
- Allows for soft tissue expansion with midstance pronation
- Distal to heel seat
- Blended into balance platform
- No plaster applied to “control point”
Lateral expansion plaster
- Allows for weight bearing soft tissue spread
- Should not extend beyond 1 cm medial of calcaneal bisector
- Addition around heel at 45°
- Fabricot smooth
Evaluation of positive
- Medial expansion plaster does not alter balance plaster
- No plaster on “control point”
- Plaster blends smoothly into anterior balance platform
Pressing preparation
- Vacuum press
- Cover positive with smooth material
- Thermoplastic material
- Heat to appropriate temperature
- Must trim excess around heel to prevent “wrinkles”
Pressing the orthotic
- Capture contour of heel
- Adequate material to support lateral expansion
- Adequate material to capture medial arch
- Medial arch contour captured
Internal heel cup height
- Higher heel cup = better control
- Normal adults 12mm
- Children 16mm
Internal heel cup higher medially
- Acts as buttress against STJ pronation in midstance
Internal heel cup higher laterally
- Acts as buttress against STJ during contact
Grinding/sanding the orthotic
- Distal length to middle of metatarsal heads
- Lateral border slight curvature onto positive
- Medial border straight for shoe gear fit
Posting elevator
- Adjusts rearfoot post height to shoe gear height
Posting elevator placement
- Division between planes in line with the long axis of calcaneus
- Runs ~ 1 cm medial to calcaneal bisection out towards 5th met head
Rearfoot post
- Controls pronation during contact
- Provides some subtalar joint control
- Two planes (medial/lateral)
Rearfoot post medial plane
- Acts as a break
Rearfoot post lateral plane
- Supports inverted position
Rearfoot post measurements
- Lateral: 4° inverted
- Medial: parallel to anterior edge of orthotic
- Supports inverted presentation to ground
- Prevents STJ pronation after full forefoot load
Functional control of orthotic on the foot
- Brings the ground up to the foot
- Orthotic plate provides control during stance
- Rearfoot post provides control during contact
