– cortical bone by definition should have a porosity LESS THAN 30% – in fact, most cortical bone has a porosity about 10%.
– cancellous bone by definition should have a porosity GREATER THAN 30%. Ie. it is AT LEAST 30% porous.
– antibiotics in cement – maximal elution in first few days, then exponential drop off; “cease to be present in significant levels at 6-8 weeks.”
– 2 grams of antibiotics in 4 grams of cement is enough – higher concentrations tend to weaken the cement.
– increase in muscle strength due to training is due to hypertrophy, and increased recruitment; there MAY be an element of hyperplasia, but most agree that the majority is from hypertrophy. A number of sources state that, particularly early, muscle strength increases by improved neuronal firing and recruitment
– “recovery of muscle strength in the initial stages of rehab are often due to improved neural firing patterns more than anything else”
– muscle hypertrophy occurs by increased numbers of myofibrils/fiber
– muscle spindles monitor tension in the muscle
– ATP in the muscle fiber is concentrated in the MYOSIN.
– muscle failure occurs with eccentric loading, with failure most often occurring at or near the myotendinous junction.
– if muscle is strained to failure but does not disrupt the vascular channels – cell death and degeneration ensues, then regeneration proceeds – the muscle fiber type formed depends on the motor nerve type.
– if muscle is strained and vessels are torn – heals with inflammation and scar (which can block regeneration). The scar is mechanically better if the muscle undergoes early motion.
– the amount of recovery seen after muscle laceration depends on the amount of scar, the regeneration across the site, and the degree of denervation of distal muscle (or its ability to be re-innervated).
– after applying a tourniquet, expect local acidosis with a drop in pH.
– tissues beneath the cuff – the compression zone
– tissues distal to the cuff – the ischemic zone
– tissues in the ischemic zone seem to tolerate up to 3 hours with little sequela
– tissues in the compression zone, however, are subject to shear forces and ischemia, particularly at the cuff-limb interface. Irreversible nerve damage directly beneath the cuff can occur within 2 hours.
– hence, the tissue beneath the cuff is much more vulnerable than the tissue distal to it.
– wider cuffs are better
– after 90 minutes of tourniquet time – need to let it down for 5 minutes to reperfuse; 15 minutes if time is 3 hours.
– sequence of collagen formation: translation, hydroxylation, and glycosylation all occur within the cell to form the soluble procollagen (a triple helix). This is then exuded OUT of the cell where the terminal peptidase is cleaved off by procollagen propeptidase and it forms the insoluble tropocollagen. The tropocollagen then crosslinks to form collagen
– hence, the procollagen is intracellular, the tropocollagen is extracellular.
– collagen breakdown is well assessed with urinary hydroxyproline (Paget’s)
– comparing fresh and deep frozen allografts: the freezing process makes it less immunogenic; freeze dried are kept at minus 30, deep frozen at minus 80; deep freezing may actually maintain some osteoinductivity, but freeze drying definitely eliminates any of the osteoinductivity; fresh allografts would probably incorporate more, but are subject to the immune response.
– the key concept is that FREEZING diminishes the immunogenicity
– interestingly, both cortical and cancellous grafts undergo creeping substitution, but in the opposite order – cancellous grafts are weak, but they are invaded by osteocytes, which lay down bone, which later is resorbed and replaced (ie. they undergo bone formation before bone resorption). Cortical grafts are strong at first, but are broken down first before new bone is laid down.
– cortical grafts are 50% of their initial strength at 6 months.
– in cartilage allografts, the chondrocytes are NOT