2018- Donna Magid, M.D., M.Ed.
Loving the Lower Extremity... and a few friendly words about Stance and Gait
NORMAL GAIT inherently symmetric, energy-efficient and balanced; requires
- Stability in stance (standing on leg- forces act at center of gravity)
- Means of progression (swinging from one stance to next stance- accelerating)
- Energy conservation
ABNORMAL GAIT therefore due to and/or produces:
- Decreased stability
- Change in joint forces and wear
- Energy insufficiency (too much work)
Altered gait = leg length discrepancy, neurologic disease, injury/surgery/pain (antalgic)
In stance weight divided between 2 legs. Static stance on one leg shifts all weight to that leg. BUT Ambulation forward dynamically magnifies forces (mass x acceleration) → 3-6x body weight and at the point the magnified weight flows from the entire upper body to the tiny surface area of the superior pole femoral head can → up to 8-10x body weight (e.g. running down steps).
- Loading increases articular surface contact areas; load-per-area can be huge (eg 340% increase superior pole femoral head early in gait →g +800% running down stairs)
- Cartilage resilient, accommodates (reshapes) and hydraulically cushions; both responses decrease with age. Loss of cartilage redistributes forces, changes angles of loading, accelerates degeneration.
LOWER EXTREMITY (LE)
BIRTH: very ‘bowed’ (varus); in utero molding and ‘absence’ of gravity. Age-related emodelling reflects weight-bearing, gravity, Wolff’s Law (See ‘Fractures 101”). Medial physis distal femur grows faster than lateral, also contributing to adult valgus LE.
“PHYSIOLOGIC BOWING” (GENU VARUM) NORMAL til late in 2d year; should resolve by 18-20 mo. female, 20-24 mo. male. Parents tend to self-refer 14-36 months with concerns.
>10 degrees varus 14-36 mo = ”Physiologic bowing” if can r/o possible pathologic etiology (trauma, hypophosphatemic or other rickets, septic arthritis and growth arrest, achondroplasia, infantile tibia varum/Blount disease, OI, NF,…). Should be symmetric, distributed through both femur and tibia (both bow medially), should resolve untreated
3-5 yrs: may show slight ‘overshoot’, with truly straight leg (actually =s ‘knock-knee’d”)
6-7 yrs: true adult pattern : (see Drawing)
- MECHANICAL AXIS (A) (long image only, usually standing; from center of femoral head→ center of notch roof → center distal tibial plafond). NORMAL: 0 degrees.
- ANATOMIC AXIS (B) (shaft-shaft at knee, standing). Normal: 6-8 degrees valgus
INFANTILE TIBIA VARA (BLOUNT DISEASE): proximal tibia grows asymmetrically bowing LE.
- Medial proximal tibial arrest/delay → lateral proximal tibia grows, medial doesn’t, pushing lower leg into medial or varus angulation. Progressive failure and deformity medially (see diagram)
- Etiology uncertain: early walkers, infant obesity, female, African-American
REPORTING LOWER EXTREMITY RADIOGRAPHS
Lower extremity radiographs provide either a stitched or Eos-type low-dose image of the bilateral lower extremities, inferior pelvis through the ankles. While the req will usually state “leg length” or “LLD” (leg length discrepancy), there are two sets of measurements to be made: MECHANICAL AXIS and LENGTH.
Start report with age and gender, particularly if skeletally immature; normal infants are bowed (varus mechanical axis, 2-3 yr olds in transition, and older children and adults should approximate a mechanical axis of zero. Right and left axes should also be symmetric. Comment if PELVIC OBLIQUITY (one side of pelvis cephalo-caudally angled relative to contralateral—which is NOT “tilt”, or angulation in AP plane). If there are obvious findings—plate and screw epiphysiodeses, osteochondromata, healing/healed fractures, rickets, etc—comment briefly.
MECHANICAL AXIS: open the ANGLE measurement tool; angles require 2 lines determined by 3 landmark points. First landmark drops in the estimated center of the (ossified) femoral head if it were a complete sphere. Second landmark goes in the mid-point of the distal femoral physis (child) or at estimated roof of notch (adult), Third landmark goes to mid plafond distal tibia. Subtract given angle from 180 for axis. If not zero, determine if axis is VALGUS (tibia long axis angles laterally relative to femur’s) or VARUS (tibial axis angles medially relative to femur).
LENGTH: Drag the angle measurements laterally, just off legs, to clear the playing field. Open the LENGTH tool.
FEMUR Landmarks are the most proximal ossified femoral head superior pole and the most distal intercondylar point mid distal articular femur. Round up anything greater or equal to .x5 to next tenth; report only one digit to right of decimal point.
TIBIA landmarks (you may have to move femur lines off femurs to clear field) are the most proximal ossified point between the tibial spines, and the most distal mid articular plafond.
Caveat: get in the habit of consistency; ie drop the little box landmark that determines the measurement line in the same way each time—try to bisect the box by centering it over chosen bony landmark. Actually takes some practice.
“This is a boy, chronologic age 5 yrs 6 months. The mechanical axes lower extremities are 2 degrees valgus right, zero degrees valgus left. No significant pelvic obliquity, no definite focal asymmetry.
The right femur measures approximately 20,4 cm, the left 20.9 cm, 5 mm longer.
The right tibia measures approximately 16.7 cm, the right 16.0 cm, 7 mm shorter; total discrepancy 2 mm”
Report the facts; one need not further opine on ‘normal/abnormal’. Occasionally I add an opinion for children in that tricky 2-3 yr period where bowing resolves (“Likely within limits for a girl 21 months old” but don’t if unsure of your judgment/experience. Remember (read the Lower Extremity hand out!) girls lose physiologic bowing slightly earlier, 20-24 months, boys 24-28 months.
2018 - Donna Magid, M.D., M.Ed.