I think that bone turnover is the bone resorption and formation processes (not only to repair damages, but a continuos process on a cellular / tissue level). While bone remodeling is bone reformatting (or reshuffle, for example its size and shape) in a organ / system level. Bone remodelling can be due to the growth or diseases.

The two terms are similar but they cannot be used interchangeably, even if various authors often confuses the two concepts. The term “bone turnover” is a generic term indicating the normal replacement of bone tissue (as in many other tissues); bone turnover consists in the replacement of approximately 10% of the skeleton every year. The bone turnover is performed by means of the process of “bone remodeling”, whose term precisely indicates cellular processes occurring in temporal sequence: first, bone erosion by osteoclasts; after, bone formation by osteoblasts, in answer to different causes (metabolism, loading, etc.) that involves microscopic change of bone structure.

You also could say that the two terms are related to each other as “cause and effect” or better “mechanism-effect”: the mechanism by which bone turnover is performed is the bone remodeling; this relation is important because the other bone process, i. e. the “bone modeling” (which otherwise implies changes in size and shape, NOT in the mere structure of bone) does not participate to the physiological bone turnover, but it has other meanings and functions.

Parfitt [1] explained beautifully the definition of ‘bone turnover’ and ‘bone remodelling’.

Basically, bone turnover refers to the total volume of bone that is both resorbed and formed over a period of time, usually expressed as percent/year. Bone turnover can be estimated by measuring relevant bone biomarkers [2]. 

Bone remodelling, on the other hand, is defined as an active process throughout the skeleton, essential for calcium homeostasis and preserving the integrity of the skeleton, through the coupled activity of osteoclasts and osteoblasts [3]. In adults, bone turnover occurs mainly through bone remodelling [4].

In a nut shell, I agree with Carla that bone remodelling refers to a dynamic physiological event (the Process or the Mechanism) that influences bone turnover (the Effect). You might find the following references useful:

[1] Parfitt AM. Bone. 2002; 30(6):807-9.

[2] Parfitt AM. Bone. 2004; 35(1):1-3.

[3] Fazzalari NL. Semin Cell Dev Biol. 2008; 19(5):467-72.

[4] Parfitt AM et al. J Bone Miner Res. 1996; 11(2):150-9.

I agree with Chun-Yi Ng, bone turnover is measured by bone markers and remodeling by histomorphometry at tissue level.

I would like to add that some authors distinguish between "bone modelling" and "bone remodelling". The abstract of "Bone modelling and remodelling" [1] provides a definition. According to it "bone modelling" is the adaption of the structure in size and shape mentioned by Gustavo Gomes Resende. While "bone remodelling" is defined as the repair of damage.

[1] Seeman E. Crit Rev Eukaryot Gene Expr. 2009;19(3):219-33.

To be more precise, three are the bone processes, that occur at different times, with different cellular involvement and different meaning.

Bone Growth (from 4th month of fetal life, after the development of ossification centers, until the end of the somatic growth – about 20-22 years of age): it consists in dimensional increase of skeletal segments by activity of osteoblasts only.

Bone Modeling (from 4th month of fetal life, after the onset of bone growth, until the end of the somatic growth – about 20-22 years of age): it consists in macroscopic changes of the shape of skeletal segments by independent activity (on different surfaces) of osteoblasts and osteoclasts.

Bone Remodeling (it appears in the 6th month of fetal life and lasts for life): it consists in microscopic changes of bone structure by coupled activity (on the same surface with temporal sequence) of osteoclasts (first) and osteoblasts (later).

This last process (bone remodeling) allows different functions (in skeleton adaptation to the mechanical and metabolic requirements); additionally, through it bone turnover is performed.

Moreover, the first two processes determine and influence the bone ARCHITECTURE, the third instead changes the microscopic STRUCTURE of bone tissue.

Bone growth, modeling and remodeling (the only three skeletal processes that can be described systematically) are all STRUCTURAL concepts.

Bone turnover, instead, is a METABOLIC concept.

Let's forget bone growth in this instance.

Bone MODELING is the structural result of an uncoupled combination of bone formation and destruction processes produced at different bone sites that can take place either or all in periosteal, endocortical and/or peritrabecular surfaces (minimodeling), with unlimited extension in surface. The outcome of bone modeling is some change in the design of a trabecular network or of a cortical shell. The global modeling process of the skeleton always involves a mass balance, which usually (not necessarily) results positive, especially in young and adult individuals, or in those who maintain a reasonable amount of physical activity (modeling is usually highly-sensitive to mechanical stimulation). During growth, modeling can determine bone growth in width as well as morphological changes in the bone's cross sections (modeling drifts), and is the process that naturally tends to correct bone deformities. Modeling decreases with age but never disappears completely. The description of the effects of any treatent on the skeleton is never complete if it does not regard the eventual changes induced in bone modeling.

Bone REMODELING is the structural result of a coupled, resorption-reposition process taking place at very tiny points (bone structural units, BSU's, do not confound with bone multicellular units, BMU's) on any of the four bone surfaces, namely, periosteal, endocortical, peritrabecular, or intracortical surfaces. The outcome of bone remodeling is the replacement of a small amount of the pre-existing bone by a comparable (neutral balance - "conservative"-mode remodeling) or a smaller (negative balance - "disuse"-mode remodeling) piece of new bone. Remodeling is seldomly observed on periosteal surfaces. On peri-trabecular surfaces, "disuse"-mode remodeling can produce trabecular thinning, perforation, or fracture. On the endo-cortical surfaces, it can lead to enhancement of the medullar cavity and/or to cortical shell thinning. On the endo-cortical surfaces, it can enhance intracortical porosity and reduce bone tissue stiffness and toughness. Bone remodeling is the only known mechanism to replace old by new bone. Sometimes remodeling is induced by the production of a microfracture, and proceeds in such a way that the bone discontinuity gets repaired ("targeted" remodeling) . Elsewhere ("stochastic" remodeling), bone remodeling activation is a less-well known process. However, bone remodeling usually increases with age and tends to yield an increasingly negative balance of bone mass, especially in post-MP women. Perhaps for this reason, remodeling is wrongly regarded as a purely deleterious process for the skeleton. The contribution of remodeling balance to serum Ca homeostasis is a matter of discussion.

Bone TURNOVER is the ABSOLUTE AMOUNT of the whole-body BONE MASS BALANCES resulting from bone formation and destruction, WHATEVER their signs and causes (growth, modeling, or remodeling). The bone turnover concept involves no less than two different interpretations concerning diagnosis of metabolic bone diseases, namely, 1. the TOTAL AMOUNT (the sum) of both bone formation and destruction in terms of bone mass (i.e., a "low" or a "high" bone turnover), which indicates the GENERAL level of activity of bone cells in the skeleton, regardless of its sign, and 2. the sign and the amount of the mass balance resulting from the algebraic sum of bone formation and destruction (WHATEVER the causes), which determines the trend of the disease to induce or not an osteopenic condition.

IMPORTANTLY: 1. a negative turnover balance affected by an excessive rate of "disuse" mode remodeling is the cause of all known osteopenias and osteoporoses, and 2. the acquisition of a positive bone turnover resulitng from increasing bone modeling is the only known mechanism that can prevent or improve any kind of osteopenia.

DO NOT CONFOUND:

a. Remodeling with modeling.

b. Remodeling with turnover.

c. Remodeling balance with bone mass balance.

d. Bone "microstructure" or "mico-architecture" (non-specific terms) with trabecular network structure, or with the micro-structure of the bone matrix. In any case, please specify, "bone tissue micro-structure" and "micro-architecture of the trabecular network", thus confusion  will be totally avoided.

e. Bone "quality" with bone "micro-structure" or "micro-architecture". There are three different bone "qualities", namely, 1. bone "tissue quality" (intrinsic stiffness and toughness of the mineralized tissue, resulting from its micro-structure), 2. bone "design quality" (comprising the architectural properties of both trabecular networks and cortical shells) and 3. "whole-bone" quality (= structural, or whole-bone stiffness, toughness and strength at the organ level, or, more directly, resistance to fracture).

Nice discussion. Sometimes is difficult to get the whole concepts found in bone biology reasearch, especially if there is so much fragmentation between the areas involved (clinical/medicine, physiology, biomechanics, mechanobiology, microanatomy, histology, paleohistology, among the most relevant).

I agree with Carla, Chun-yi and the argentinian. But you guys are definig terms, and i think this is different, and it must be viewd in a whole sense. There are some basic confusions.

Bone modeling and remodeling are different processes, this is, real biological processes, although sometimes it is difficult to know which one is occurring at specific areas of the bone, especially if we consider different bones and/or species. However, turnover IS NOT a process, but can occurs during modeling and remodeling. BONE TURNOVER is a MEASUREMENT of bone tissue dynamics (like a rate of mineral/matrix interchange&recuperation). Chun-yi gets the concept as me ''modeling and remodeling = processes or mechanism; turnover = effects, or the quantifiable way to measure modeling and remodeling. Likewise, both modeling and remodeling are physiological processes with microanatomical (histological and geometric) effects.

The LITERATURE CONFUSES SOMETIMES SINCE IT IS MOSTLY BASED ON HUMAN BONE BIOLOGY, WHICH IS unique IN VERTEBRATES, SO SOME TERMS ARE TOO ATTACHED TO THE SPECIFIC PATTERN SHOWN BY US, WHICH LIMITITATES THE PHENOMENOLOGY OF THE BONY PROCESSES.