Discussion

The incidence and mechanism of TOH are unknown. Some women may have a low peak bone mass or be in an accelerated bone resorptive state prior to conception, and pregnancy allows it to become clinically obvious. In some reported cases, there are other secondary causes of osteoporosis including chronic heparin, anticonvulsant or corticosteroid use, suggesting pregnancy may not be solely to blame.1 Our patient was also on regular antipsychotic medication, and these have been linked to hyperprolactinaemia which can lead to reduced bone mineral density (BMD) and osteoporosis.2 There have also been documented cases of TOH where the female child resulting from the pregnancy has been found to have low BMD in childhood. This suggests there may be a genetic element. This was our patient’s first pregnancy; however, whether increased parity has a negative or beneficial impact on BMD or risk of fracture is inconclusive. In our patient, the combination of her antipsychotic medication, diabetes and her genetic composition is all likely to have contributed to this complication of TOH.

Pregnancy and lactation create a change in bone metabolism. Calcium is required for fetal growth and lactation. The increased calcium demand on top of hypoestrogenaemia during pregnancy can lead to bone mineral loss and is therefore contributory in the development of TOH. One study found a 7.6% bone density loss at the lumbar spine immediately post partum compared with age-matched, non-postpartum controls.3 To compensate for the raised calcium demand, particularly during the third trimester, the intestinal absorption of calcium during pregnancy doubles.4 However, studies found that all markers of bone turnover increased during pregnancy and failed to return to normal baseline levels by 12 months post partum, suggesting maternal skeletal stores are also depleted to compensate.5 Additionally, the change in hormone levels such as progesterone, oestrogen and relaxin can lead to ligament laxity. The 10–15 kg weight gain can increase bone and joint loading during everyday activities. Women often undergo less physical activity during pregnancy, particularly during the third semester.6 Additionally, conditions such as pregnancy-induced hypertension or preterm labour can lead to women being on long-term bed rest. These factors can all contribute to increasing the risk of stress or fatigue fractures during labour.

As lactation increases calcium demands and produces hormonal changes, studies have looked into lactation-related bone losses, but the results vary substantially. Some studies suggest a >5% loss of BMD within the spine due to breastfeeding, but the quantity of bone loss is positively associated with the volume of breast milk produced. However, this loss appears to be transient as there is full recovery following cessation of lactation. Alongside weaning, return of menstruation and use of progesterone-only pill were associated with improved recovery.6

Fragility fractures often occur in the first pregnancy, commonly in the third trimester or in the first month post partum, often in the spine or hips. Patients most frequently present with the multiple vertebral fractures. The increased BMD loss in the spine compared with the hip is thought to be a result of bone composition: vertebrae are rich in trabecular bone which has a higher turnover rate, while the hip has more cortical bone.7 This is reflected in a study comparing BMD loss in hip and spine during pregnancy where the T-scores and Z-scores were higher at the hip than the spine regardless of maternal age, parity and/or lactation history.8 Our patient underwent a bone scan postoperatively which only included the lumbar spine due to the bilateral hip implants, and therefore a comparison of bone density between hip and spine was not possible.

Early diagnosis is key to prevent serious complications and should be considered in pregnant patients presenting with persistent hip or lower back pain. If suspected, MRI is the imaging modality of choice. Identifying fractures in the acute stage before they become displaced or result in avascular necrosis affects the long-term impact on the patient. Any fracture identified will require an operation to internally fix the bone or even replace the joint altogether if avascular necrosis is a risk.

There are currently no guidelines in terms of prevention; lifestyle changes are a good first step: sufficient exposure to sunshine for vitamin D and regular exercise to help strengthen bones. Vitamin supplements can also play a key role.7 The current guidance for daily calcium intake for non-pregnant individuals is 700 mg.9 Recommendations for daily vitamin D is 400 IU and includes those who are pregnant.10 The condition can be pharmacologically managed with bisphosphonates. One study has shown a substantial increase of 23% in spinal BMD in response to early bisphosphonate therapy alongside calcium and vitamin D supplementation. In individuals without bisphosphonate therapy, the spinal BMD increase was 11%.11 There is no clear optimal duration for bisphosphonate therapy following birth, although 5 years seems to be widely accepted.4 11 However, there are concerns about bisphosphonates collecting within the maternal bone and being exposed to the fetus of subsequent pregnancies, affecting the fetal skeletal development.4 Teriparatide, a parathyroid hormone treatment, has been recently proposed as an alternative treatment. It has been shown to increase BMD and reduce the risk of osteoporotic fracture. Following 13 months of treatment with teriparatide, one study demonstrated a 24.4% increase in lumbar spine BMD and 12.6% and 7.8% at the left and right neck of femur, respectively.12 Furthermore, it does not accumulate in the maternal skeleton and does not affect a future fetus after cessation of therapy.

A previous case report documented a woman of similar age to our patient who presented with increasing pain in her right leg at 30 weeks’ gestation. On presentation, she underwent an MRI scan which revealed TOH and a stress fracture. To reduce the risk of hip fracture, the patient was put on bed rest for the remainder of her pregnancy and underwent a caesarean section to avoid a vaginal birth. Postnatally, the patient was given vitamin D replacement and bisphosphonates; by 6 months post partum, her symptoms and MRI changes had resolved. This case demonstrates the importance of early recognition of TOH.13

In summary, women with TOH often suffer severe and disabling back/hip pain. Exact mechanisms involved are still debated, and there are many factors that increase the risk of reduced BMD during the pregnancy and lactation period. Having a low index of suspicion in patients with disproportionate pelvic or hip pain will allow early identification of possible fractures. MRI scanning is the recommended method of investigation during pregnancy. MRI is safe during the second and third trimesters of pregnancy, and while consideration should be given to use in the first trimester, there are no associated long-term complications.14 This is vital to initiate treatment promptly and reduce the risk of complications, such as avascular necrosis.