If there are studies, then link them. Until then, I claim the magic 5 cm to be no more, than speculation.
Common sense is that the less you lengthen then the chances of better recovery and lower complications is higher. In terms of safe distance lengthening i will quote the study i posted a while ago.
Effects of bone lengthening on surrounding tissues
Gradual bone lengthening has a negative impact on the surrounding muscles through stretch, impalement by pins and wires, pain and inflammation. The speed of the gain in length during callotasis with a 1-mm/day rate is about four to eight times faster than during the adolescent growth spurt with its temporary muscle shortenings [8]. No wonder that the clinical challenges of muscle distraction and subsequent decrease of adjacent joint range of motion go in parallel with the amount of lengthening. This is reflected by histological changes after lengthening of more than 30 % of its original length [22, 116–119], as the whole muscle from origin to insertion is stretched [81]. The elastic limit of stretched muscles (strength–strain curve) is 10–15 % of the length at rest. Excessive stretch leads to plastic deformation and subsequent contractures, which commonly affect muscles spanning two joints (rectus femoris, hamstrings). There are only few data about the loss of muscle power during the distraction–consolidation process and the speed and amount of recovery thereafter. There is a small residual decrease in muscle strength and power after surgical lengthening without any impact on the activities of daily living [120]. Pre-operative muscle training as a preventive measure and a post-operative intense rehabilitation programme including continuous passive motion, extension splint, strengthening and stretching exercises, as well as proprioceptive training, are mandatory until the pre-operative level is reached [66]. Nerves and vessels adapt in length during the distraction process and recover from temporary degenerative changes within 2 months after the halting of distraction [104]. Excessive gradual (>20–30 %) or acute distraction (>15 %) may both lead to partial or complete loss of nerve potentials [121, 122].
Joint cartilage may be exposed to reactive forces which increase linearly with distraction [123]. In addition, non-weight-bearing and decreased range of motion diminishes nutrition of the cartilage and may support histological changes which appear after 30 % lengthening in animal experiments [124, 125].
Physeal cartilage shows experimental histopathological changes secondary to increased axial load [124]. However, alterations of the growth rate were never observed in a clinical setting [126].
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3364349/
This means that people with longer legs are able to lenghten more. But then if you lengthen lower then 15% of the original bone length then it is much more safer. Simple common sense!
Here is another study by paley that shows that for each 1cm gained by lengthening along the femur anatomical axis (internal nailing) then there is a shift of 1mm in the mechanical axis. This could increase the chance of arthritis in the future.
The effect on mechanical axis deviation of femoral lengthening with an intramedullary telescopic nail.
Authors
Burghardt RD, Paley D, Specht SC, Herzenberg JE.
Journal
J Bone Joint Surg Br. 2012 Sep;94(9):1241-5. doi: 10.1302/0301-620X.94B9.28672.
Affiliation
Abstract
Internal lengthening devices in the femur lengthen along the anatomical axis, potentially creating lateral shift of the mechanical axis. We aimed to determine whether femoral lengthening along the anatomical axis has an inadvertent effect on lower limb alignment. Isolated femoral lengthening using the Intramedullary Skeletal Kinetic Distractor was performed in 27 femora in 24 patients (mean age 32 years (16 to 57)). Patients who underwent simultaneous realignment procedures or concurrent tibial lengthening, or who developed mal- or nonunion, were excluded. Pre-operative and six-month post-operative radiographs were used to measure lower limb alignment. The mean lengthening achieved was 4.4 cm (1.5 to 8.0). In 26 of 27 limbs, the mechanical axis shifted laterally by a mean of 1.0 mm/cm of lengthening (0 to 3.5). In one femur that was initially in varus, a 3 mm medial shift occurred during a lengthening of 2.2 cm. In a normally aligned limb, intramedullary lengthening along the anatomical axis of the femur results in a lateral shift of the mechanical axis by approximately 1 mm for each 1 cm of lengthening.
http://www.ncbi.nlm.nih.gov/m/pubmed/22933497/So the conclusion is very simple, the lower you lengthen the higher chance of safe recovery and a lower risk of long term complications.
Author
Topic: Strongly considering LL surgery - been obsessed with height since late teens (Read 57832 times)