Theories on How Strontium Prevents Fractures

Physiochemical Mechanisms/Sacrificial Bonds 
Physiochemical mechanisms describe "how Sr (strontium) is able to affect the intrinsic tissue quality of bone by directly incorporating into bone at multiple levels, including the organic matrix and the hydrated layer surrounding hydroxyapatite (HA) crystals, forming sacrificial bonds and stabilizing hydration shells." 

Cellular Mechanisms/Ca-sensing Receptor (CaSR) 
"The cellular mechanisms are centered mainly around the Ca-sensing receptor (CaSR) and its ability to mediate bone cell function and respond to Sr, providing evidence for the dual-acting mechanism."

"Osteoclast precursors and mature osteoclasts express the CaSR on their cell membrane..."

Enhanced Bone Microarchitecture 
"Microarchitecture is an important aspect of bone strength and increased fracture risk in patients with osteoporosis is associated with trabecular thinning and increased cortical porosity."

"Clinical studies have shown SR treatment increased cortical thickness and trabecular number and decreased trabecular spacing from baseline, determined using high-resolution peripheral quantitative computed tomography (HR-pQCT)." 

https://www.sciencedirect.com/science/article/pii/S2352187220300334

Vegetable Magnesium Stearate in Strontium Citrate Products

Magnesium stearate, also known as vegetable stearate, is used as a "flow agent" in supplement tablets and capsules to prevent different supplement ingredients from sticking to each other and to the blending and punching equipment. The flow agent ensures a homogenous blend of ingredients and a consistent dosage in each and every capsule or tablet. It is a common ingredient in many supplements, including strontium citrate products. I have never had a problem with it, but some people might have. 

"Magnesium stearate is generally safe to consume, but too much of it can have a laxative effect. In large amounts, it can irritate the mucus lining of the bowels. This may trigger a bowel movement or diarrhea."

"Allergic reactions to magnesium stearate are rare, but you should avoid it and talk to your doctor if it affects you."

https://www.webmd.com/vitamins-and-supplements/what-is-magnesium-stearate

Below is a list of strontium citrate products with and without magnesium stearate. It is not an exhaustive list. If your product is not listed here, check the label. 

Strontium Citrate Products with Magnesium Stearate

Life Extension Strontium has microcrystalline cellulose, vegetable cellulose (capsule), vegetable stearate, and silica, in addition to strontium citrate. 

Nova Nutritions Strontium Citrate has a gelatin capsule, rice powder, and vegetable magnesium stearate.

Vitacost Strontium has hydroxypropyl methylcellulose, microcrystalline cellulose, vegetable stearic acid and vegetable magnesium stearate in addition to strontium citrate. 

Strontium Citrate Products without Magnesium Stearate but with Other Additives

Swanson Strontium Citrate has gelatin, rice flower, calcium laurate, and silica.

Pure Encapsulations Strontium (Citrate) has a vegetarian capsule (cellulose, water) and contains hypoallergenic plant fiber (cellulose) and ascorbyl palmitate. 

Strontium Citrate Products with No Additives

AOR Strontium Support II, which I take, is in a capsule composed of hypromellose and purified water and contains no additives.

AlgaeCal Strontium Boost is in a vegetable cellulose (vegetarian capsule) and contains no additives.

Strontium Chloride

I just became aware of a strontium chloride product for bone health,
Eniva Strontium Mineral Liquid Concentrate. One fluid ounce (30 mL) contains 250 mg strontium. The ingredients are purified water, strontium (from strontium chloride), natural flavors, citric acid, sorbic and/or benzoic acid(s) [protects freshness], stevia leaf extract. A liquid may offer quicker absorption than tablets or capsules that must first dissolve in the digestive tract before being absorbed. 

https://eniva.com/strontium-mineral-liquid-concentrate-32-oz/?sk...